My situation is I have two images with the same tag(hash different), one at local and another at the registry. When I build dockerfile, docker always compares the hash of the two images find not equal then will pull the registry one.
I know there has an imagePullPolicy in k8s. My question is docker has any settings like imagePullPolicy?
Thanks a lot.
The Docker tooling by and large either assumes you're going to manually pull images, or provides a --pull option to integrate it with other commands. For example:
docker build has a --pull option to try to retrieve a newer version of FROM images
docker run does not; it will always reuse the image you already have, or pull one if you don't have it
Neither core docker-compose nor docker-compose up has a --pull option, but there is a docker-compose pull command that pulls every image listed in a docker-compose.yml file
docker-compose build does have a --pull option
Core Docker always tries to pull an image if it is not present; there is no equivalent to imagePullPolicy: Never. Conversely, it never tries to communicate with an image registry outside of an explicit "pull" operation; you also cannot make docker run act like imagePullPolicy: Always.
It's good practice in Kubernetes to use a unique tag per build, so you can specify an explicit build and don't have to worry about imagePullPolicy. If you do this, in plain Docker, the implicit "pull if missing" will get you the correct behavior as well.
As a potential addendum to #David Maze's answer, it looks like docker run now also has a --pull option.
Related
I'm running into some unintuitive behavior with docker in a CI/CD system. I'm specifically trying to improve the performance of docker builds when the docker layer cache is empty.
I've found the --cache-from argument for the docker build command which is super useful. What I'm having trouble wrapping my head around is why it seems like that flag is needed if I perform a docker pull prior to running docker build.
My intuition (which is obviously wrong) tells me that docker pull should populate the layer cache make make the --cache-from argument for building unnecessary. However, I'm finding that even if I perform docker pull I still need to use the --cache-from flag to use the layer cache when performing builds in my CI/CD system. Why is that?
I have a Git repo with a simple Dockerfile. First row goes like this:
FROM python:3.7
My company has an internal registry with the base images. Because of this, the DevOps guys want me to change the Dockerfile to:
FROM registry.company.com:5000/python:3.7
I don't want this infrastructure detail baked in my code. URLs may change, I may want to build this image in another environment, etc. If possible, I would rather indicate the server in the pipeline, but the documentation regarding docker build has no parameter for this.
Is there a way to avoid editing the Dockerfile in this situation?
You would use a build arg for this:
ARG registry=docker.io/library
FROM ${registry}/python:3.7
Then for the build process:
docker build --build-arg registry=registry.company.com:5000 ...
Use docker.io for the registry name for the default Docker Hub, and library is the repository for official docker images, both of which you normally don't see when using the short format. Note that I usually include the library part in the local mirror so that official docker images and other repos that are mirrored can all use the same registry variable:
ARG registry=docker.io
FROM ${registry}/library/python:3.7
That means your local registry would need to have registry.company.com:5000/library/python:3.7.
To force users to specify the registry as part of the build, then don't provide a default value to the arg (or you could default the value of registry to something internal if that's preferred):
ARG registry
FROM ${registry}/python:3.7
You can work around the situation by manually pulling and re-tagging the image. docker build (and docker run) won't try to pull an image that already appears to be present locally, but that also means there's no verification that it actually matches what Docker Hub has. That means you can pull the image from your mirror, then docker tag it to look like a Docker Hub image:
docker pull registry.company.com:5000/python:3.7
docker tag registry.company.com:5000/python:3.7 python:3.7
I'm in low cost project that we send to container registry (DigitalOcean) only latest image.
But all time, after running:
docker build .
Is generating the same digest, every time.
This is my script for build:
docker build .
docker tag {image}:latest registry.digitalocean.com/{company}/{image}:latest;
docker push registry.digitalocean.com/{company}/{image}
I tried:
BUILD_VERSION=`date '+%s'`;
docker build -t {image}:"$NOW" -t {image}:latest .
docker tag {image}:latest registry.digitalocean.com/{company}/{image}:latest;
docker push registry.digitalocean.com/{company}/{image}
but not worked.
Editing my answer, what David said is correct - the push with out the tag should pick up latest tag.
If you provide what you have in your local repository and the output of the above commands, it would shed more light to your problem.
Edit 2:
I think I have figured out on why:
Is generating the same digest, every time.
This means, although you are running your docker build - there has been no change to the underlying artifacts which are being packaged into the image and hence it results into the same digest.
Sometimes layers are cached but there are changes that aren't detected so you can delete the image or use 'docker system prune' to force clearing cache here
I have a use case where I call docker build . on one of our build machines.
During the build, a volume mount from the host machine is used to persist intermediate artifacts.
I don't care about this image at all. I have been tagging it with -t during the build and calling docker rmi after it's been created, but I was wondering if there was a one-liner/flag that could do this.
The docker build steps don't seem to have an appropriate flag for this behavior, but it may be simply because build is the wrong term.
When building a docker image you normally use docker build ..
But I've found that you can specify --pull, so the whole command would look like docker build --pull .
I'm not sure about the purpose of --pull. Docker's official documentation says "Always attempt to pull a newer version of the image", and I'm not sure what this means in this context.
You use docker build to build a new image, and eventually publish it somewhere to a container registry. Why would you want to pull something that doesn't exist yet?
it will pull the latest version of any base image(s) instead of reusing whatever you already have tagged locally
take for instance an image based on a moving tag (such as ubuntu:bionic). upstream makes changes and rebuilds this periodically but you might have a months old image locally. docker will happily build against the old base. --pull will pull as a side effect so you build against the latest base image
it's ~usually a best practice to use it to get upstream security fixes as soon as possible (instead of using stale, potentially vulnerable images). though you have to trade off breaking changes (and if you use immutable tags then it doesn't make a difference)
Docker allows passing the --pull flag to docker build, e.g. docker build . --pull -t myimage. This is the recommended way to ensure that the build always uses the latest container image despite the version available locally. However one additional point worth mentioning:
To ensure that your build is completely rebuilt, including checking the base image for updates, use the following options when building:
--no-cache - This will force rebuilding of layers already available.
The full command will therefore look like this:
docker build . --pull --no-cache --tag myimage:version
The same options are available for docker-compose:
docker-compose build --no-cache --pull
Simple answer. docker build is used to build from a local dockerfile. docker pull is used to pull from docker hub. If you use docker build without a docker file it throws an error.
When you specify --pull or :latest docker will try to download the newest version (if any)
Basically, if you add --pull, it will try to pull the newest version each time it is run.