Let the parent Docker some_img have argument X set to '0'.
How to override the argument value and have the parent Docker use other X?
I tried
ARG X='1'
FROM some_img:latest
...do stuff...
but clearly some_img still uses X='0' because ARG='0' is specified in some_img Dockerfile. ENV would take precedence over ARG, but this is not allowed before FROM.
Note that if I were just to build some_img, I could override ARG in Dockerfile by using --build-arg option in docker build.
Is there any way to override arguments in parent Docker?
No, there's no way to do this.
The result of docker build is an opaque, immutable image. A downstream Dockerfile's FROM line doesn't rebuild the image, it simply imports that opaque image as-is. You can't change what's in the image or its build options after it's been built, or cause it to be rebuilt in the later Dockerfile.
If you control both images, it's possible the thing you're trying to change shouldn't be a build-time ARG at all. Things like upstream package version numbers can be good ARGs (a Java 8 vs. Java 11 version of something is likely "different" enough to merit two builds but still be quite similar on the whole; updating from version 3.4.1 to 3.4.2 of some library probably only involves changing a version number but not any logic). Things where you need to control the behavior later on (numeric user IDs, database locations, ...) should be configured some other way and not built in to an image.
Related
Let's say I need A.exe and B.exe installed in my final node image by runtime. Both A.exe and B.exe happen to be available on Docker Hub, but they're from separate images. Does Docker have a way to somehow make both executables from different images available in my final image?
I don't think Docker's multi-stage build is relevant as it only simplifies passing artefacts that we want available on the next image. Whereas in my case, I need the whole runtime environment from previous images to be available. I have the option to RUN shell commands to manually install these dependencies but is this really the only way?
You could use a multistage build, since you could declare a.exe/b.exe together with the required runtime to be the required artefacts.
But I agree it could be easier if you install the runtime from packages and just copy the application.
If you feel sure enough about what you need, you could export image A and image B as tar balls.
Now comes the tricky part: merging the two filesystem structures
Extract both archives such that you have one target filesystem structure and wrap that up into a tar ball again.
So it is not impossible to get the files - but you need to know exactly what you are doing.
Finally import that tar ball into a docker image.
One option to get a combined image is not to merge two images but maybe merge two dockerfiles. This may need a review once in a while but might even change less often than the images themselves.
A year ago AWS announced support for Cotainer Images. The idea is that you could run your Docker image in Lambda. The image could be created based on one of the provided base images, or completely from scratch. With the first approach one just have to implement a handler, just like with regular Lambda function, while image build with the second approach need to implement Lambda Runtime API.
Building images based on the provided base images is simpler, and the documentation gives an example for Gradle:
FROM public.ecr.aws/lambda/java:11
# Copy function code and runtime dependencies from Gradle layout
COPY build/classes/java/main ${LAMBDA_TASK_ROOT}
COPY build/dependency/* ${LAMBDA_TASK_ROOT}/lib/
# Set the CMD to your handler (could also be done as a parameter override outside of the Dockerfile)
CMD [ "com.example.LambdaHandler::handleRequest" ]
task copyRuntimeDependencies(type: Copy) {
from configurations.runtimeClasspath
into 'build/dependency'
}
build.dependsOn copyRuntimeDependencies
But I'm already using Gradle Shadow Plugin and I don't like the idea of introducing a task just to copy dependencies. It looks like a hack.
Can I use that shadow JAR to build Java AWS Lambda Container image?
It turns out you can! Skip copying the classes to $LAMBDA_TASK_ROOT completely, and copy only a single fat shadow jar to the $LAMBDA_TASK_ROOT/lib:
FROM amazon/aws-lambda-java:latest
COPY build/libs/build-all.jar ${LAMBDA_TASK_ROOT}/lib/
CMD [ "com.example.LambdaHandler::handleRequest" ]
I am wondering if it is possible to swap out a layer of a container image for another. Here is my scenario:
I have a docker file that does the following:
It pulls the .net core 3.1 debian slim (buster) image
Builds the source code of my application and adds it as a layer on top of the .net core image
When a new version of the .net core 3.1 runtime image comes out, I would like to make a new image that has that new version, but has the same application layer on top of it.
I do NOT want to have to find the exact version of the code I used to build the application and re-build.
The idea is to replicate upgrading the machine and runtime, but not have any alteration to the application (less to test with the upgrade).
Is there a docker command that I can use to swap out a layer of an image?
This answer is broken into two distinct sections:
Part I: Build From Sources Instead: Explains why this idea should not be done.
Part II: A Proof-of-concept Implemented CLI Tool: A CLI tool I implemented specifically for this question, with a demonstration.
The first section can be skipped if potentially adverse side effects are not a concern.
Part I: Build From Sources Instead
No, there is probably not a docker command for this - and likely never will, due to a whole slough of technical issues. Even by design, this is not meant to be trivially possible; images and layers are meant to be immutable after being built. It is strongly recommended that the application image is instead rebuilt from the original sources, but with a new base image by modifying the FROM command.
There are numerous consistency issues that make this idea ill-advised, of which a few are listed below:
Certain Dockerfile commands do not actually create a layer, they update the final layer's internal manifest and the image's manifest. Commands that state Removing intermediate container XXXXXXXXXXXX are actually updating these aforementioned manifests, and not creating new layers. This requires correctly updating the only relevant changes when swapping from the old base image to the new base image; e.g. reconciling changes from ENV/LABEL/MAINTAINER/EXPOSE/CMD/ENTRYPOINT commands.
ENV commands that alter the application image's configuration from variables inherited in the previous base image may not be updated correctly. For example, in the application image, there might be the following command ENV command:
ENV PATH="/path:${PATH}"
If the application image's old base image layers are swapped out, and the new base image contains a different ${PATH} variable, it is ambiguous how to reconcile the differences without a manual decision by a developer.
If apt/apt-get/apk/yum is used to install Linux packages, these packages are installed in the application image as subsequent layers. The installed packages are not guaranteed to be compatible with the new base image if the underlying OS or executables change in the new base image layers.
Part II: A Proof-of-concept Implemented CLI Tool
"Upgrading" an image's base is technically possible by doing direct manipulation on the image archive of the already-built Docker application image. However, I cannot reiterate this enough - you should not even be attempting to edit the bottom layers of an existing image. Seriously - stop it, get some help. I am 90% sure this is a war crime.
For the sake of humoring this question thoroughly though, I have developed a proof-of-concept CLI tool written in Java over on my GitHub project concision/docker-base-image-swapper that is designed to swap base images (but not arbitrary layers). The design choices I made to resolve various consistency issues are a "best guess" on what action should be taken.
Included is a demonstration for swapping the base image for an already-built Java application image from JDK8 to JDK11, implemented in the demo/demo.sh script. All core code is ran in isolated Docker contains, so only Bash and Docker are necessary dependencies on the host to run this demonstration. The demo application image is only built once on JDK 8, but is run twice - once on the original JDK 8 image, and another time on a swapped-base JDK 11 image.
If you experience some technical difficulty with the tool, I may potentially be able to fix the issue. This project was quickly hacked together and has pretty poor code quality; furthermore, it likely suffers from various unaccounted edge cases. I might thoroughly rewrite this in Rust within the next month or two with a focus on maintainability and handling all edge cases.
Warning: I still do not advise trying to edit images; the use of the tool is at your own risk.
The Concept
There are three images relevant in this process, all of which are already-built (i.e. no original sources are needed):
application image: the application image that is currently based off of the old base image, but needs to be swapped to a newer base image.
old base image: the older base image that the application image is based off using a FROM command in the original Dockerfile.
new base image: the newer base image that should replace the application image's layers inherited solely from the FROM old-base-image layers.
By knowing which layers and configurations of the application image are inherited from the old base image, they can be replaced with the layers and configurations from the new base image. All image layers and manifests can be obtained as a tar archive by using the docker save command. With an archive(s) of all relevant three images, a tool can analyze the differences between
Warnings on Alternatives
Beware of doing simply a COPY --from=... from the old application image, as the original application's image configuration (through commands such as CMD, ENTRYPOINT, ENV, EXPOSE, LABEL, USER, VOLUME, WORKDIR) will not be properly replicated.
Two tools that provide this type of functionality that I have found are listed below.
Crane rebase https://github.com/google/go-containerregistry/blob/main/cmd/crane/rebase.md#using-crane-rebase
Buildpack (rebase) https://buildpacks.io/docs/concepts/operations/rebase/
NOTE: You do have to watch that you don't have items that need updates in higher layers. And that higher layers do NOT have updates in them that would conflict with the new base. Example using image below the base is a TOMCAT container. If layer 4 of the original container did a update tomcat package that would overshadow the update of same files on the NEW BASE. Which could result in a non functional system. So as always your mileage may vary.
Is it possible? Yes. But it's rarely done because of how error prone it is. For example, if the previous build would have created a library, but the library already exists in the original base image, that library won't be included in the first build. If the base image removes that library, then the resulting merged image will be missing that library since it's not in the new base layers and isn't in the old application layer.
There are a few ways I can think of to do this.
Option 1: If you know the specific files in one image, then use the COPY --from syntax to copy those files between images. This is the least error prone method, but requires that you know every file you want to include. The resulting Dockerfile looks like:
FROM new_base
# if every file is in /usr/local/bin:
COPY --from=old_image /usr/local/bin/ /usr/local/bin/
Option 2: you can export the images and create your own new image by combining the layers between the two. For this, there's docker save and docker load. That would look like:
docker save old_image >old_image.tar
docker save new_base >new_base.tar
mkdir old_image new_base new_image
tar -xvf old_image.tar -C old_image
tar -xvf new_base.tar -C new_base
cp old_image/*.json new_image/
# manually: copy each layer directory you want to save from old_image, you can view the nested tar files to find the proper ones
# manually: copy each layer directory from new_base into new_image
# manually: modify the new_image/manifest.json to have the new tag for your image, and adjust the layer references
tar -cvf new_image.tar -C new_image .
docker load <new_image.tar
Option 3: this could be done directly to the registry with API calls. You would pull the old manifest, adjust it with the new layers, and then push any new layers and then the new manifest. This would require a fair bit of coding (see regclient/regclient for how I've implemented some of these API's in Go).
Option 4: I know I've seen a tool that does this in a specific scenario but the name of it escapes me. I believe it required that you use their base images that were curated to reduce the risk of incompatibilities between versions, and limited what base images could be swapped, so I don't think it was a general purpose tool.
Note that option 2 and 3 both require either manual steps or for you to write some code if you want to automate it. Because of how error prone this is (as described above) I don't think you'll find anyone maintaining and supporting a tool to implement it. The vast majority rebuild from a Dockerfile using a CI tool.
I have developed a small utility script using Python which lets you append a tarball to an existing container image in a container registry (without having to pull existing image data), available at https://github.com/malthe/appendlayer.
The following illustrates how it works:
[ base layer ] => image1
[ base layer ] [ appended layer ] => image2
The script supports any registry that implements the OCI Distribution Spec.
Consider the following scenario. There is an application that depends on the libraries "A", "B", "C", to build and run, otherwise it throws an error. Not knowing about the dependencies "B", and "C" a Dockerfile is created that builds an image with the dependency "A" installed.
The app is run inside a container started from the image and the app fails to build, since the container is missing the dependencies "B" and "C".
Now if the image is destroyed and rebuilt, the previously downloaded dependencies will again be re-downloaded. A workaround could be to write a Dockerfile to import from the existing image (that has the dependency "A" installed) and mention the installation of the dependencies "B" and "C" on top of it.
But this way, Every-time a new dependency needs to be added a new docker image has to be built that will import from the old image, so, the old and the new image both remains important.
My question is that
if there is any way to keep building images mentioning the new dependencies without re-downloding the old dependencies?
without importing the dependencies from the old image ?
and, without the hassle of writing a new "FROM" in the dockerfile?
What is the most clean solution for a scenario like this?
1. If there is any way to keep building images mentioning the new dependencies without re-downloading the old dependencies?
Well, i often optimize Dockerfile using layer caching. Whenever you write down a command in Dockerfile, it creates a new layer. Between 2 times build, docker compares the Dockerfile's commands top down and rebuild from where it detects command's changes. So i often put stable layers (like dependencies, environment setup) at the top of dockerfile. Otherwise layers like EXPOSE Port or CMD which i often change so i put them at bottom of the file. By doing this, it saves a lot of time whenerver i rebuild image.
You can also use multistage-build. But i not often use it so you can check it here: https://docs.docker.com/develop/develop-images/multistage-build/
2. without keeping the old image and import from that into the new one?
Sometime when i want to reinstall everything again, i just rebuild image use option --no-cache.**
docker build --no-cache=true .
3. Without the hassle of writing a new "FROM" in the dockerfile
Sometimes i use base image like linux alpine and install everything i need from scratch so my image will have smaller size and does not contain things that i dont need. FROM is just pulling images from Dockerhub which are created by the some way.
For example Dockerfile of image nginx-alpine :
https://github.com/nginxinc/docker-nginx/blob/2ef3fa66f2a434cd5e44e35a02f4ac502cf50808/mainline/alpine/Dockerfile
You can checkout alpine linux for more details: https://alpinelinux.org/
I'm learning to use Docker and I've come across a minor annoyance. Whenever I make a change to the Dockerfile,I run docker build -t tag . which goes through the entire Dockerfile as it should. This takes a good 5-6 minutes due to the dependencies in my project. Sometimes a command that I run will cause an error, or there will be a mistake in the Dockerfile. While the fix may take a couple seconds, I have to rebuild the entire thing which decreases my productivity. Is there a way to "continue from where the build last failed" after editing the Dockerfile? Thanks.
This is called the "build cache" and it is already a feature of Docker. Docker's builder will only use the cache up until the point where your Dockerfile has changed. There are some edge cases when using COPY or ADD directives that will cause the build cache to be invalidated (since it hashes files to determine if any have changed, and invalidates the cache if so). This means that if you are using COPY foo /foo and you have changed that file, the build cache will be invalidated. Also, if you do COPY . /opt/bar/ (meaning, you copy the entire directory to somewhere), even some small change like a Vim swap file or Dockerfile change will invalidate the cache!
The behavior of not using the build cache at all is invoked using --no-cache in your docker build command.
So basically, it's there, and you're using it, just that you're probably changing the Dockerfile at a very early point or hitting that lesser known edge case with a COPY/ADD directive, and the builder is invalidating everything after that point. And just to answer the question before you ask it, it would be very hard or impossible to continue using the cache after a change has invalidated the cache. Meaning, if you change your first Dockerfile line and invalidate the build cache, it is basically impossible to use the build cache past that point.
Is there a way to "continue from where the build last failed" after editing the Dockerfile?
No (as L0j1k's answer explains well)
That is why the best practice is to organize your Dockerfile from the stablest commands (the one which will never have to be changed/modified) to the most specific commands (the ones you might have to change quite a bit).
That way, your modifications will trigger only a build on the last few lines of your Dockerfile, instead of going through everything again, because you changed one of the first lines.