A beginner's question; how does Docker handle underlying operating system variations when using the RUN command?
Let's take, for example, a very simple Official Docker Hub Dockerfile, for JRE 1.8. When it comes to installing the packages for java, the Dockerfile uses apt-get:
RUN apt-get update && apt-get install -y --no-install-recommends ...
To the untrained eye, this appears to be a platform-specific instruction that will only work on Debian-based operating systems (or at least ones with APT installed).
How exactly would this work on a CentOS installation, for example, where the package manager would be yum? Or god forbid, something like Solaris.
If this pattern of using RUN to fork arbitrary shell commands is prevalent in docker, how does one avoid inter-platform, or even inter-version, dependencies?
i.e. what if the Dockerfile writer has a newer version of (say) grep than I do, and they've used some new CLI flag that isn't available on earlier versions?
The only two outcomes from this can be: (1) RUN command exits with non-zero exit code (2) the Dockerfile changes the installed version of grep before running the command.
The common point shared by all Dockerfiles is the FROM statement. It is the first line in the file and indicates the parent Docker image you're building on. A typical base image could be one with Ubuntu (i.e.: https://hub.docker.com/_/ubuntu/). The snippet you share in your question would fit well in an Ubuntu image (with apt-get) but not in a CentOS image.
In summary, you're installing docker in your CentOS system, but you're building a Docker image with Ubuntu in it.
As I commented in your question, you can add FROM statement to specify which relaying OS you want. for example:
FROM docker.io/centos:latest
RUN yum update -y
RUN yum install -y java
...
now you have to build/create the image with:
docker build -t <image-name> .
The idea is that you'll use the OS you are familiar with (for example, CentOS) and build an image of it. Now, you can take this image and run it above Ubuntu/CentOS/RHEL/whatever... with
docker run -it <image-name> bash
(You just need to install docker in the desired OS.
Related
I'm trying to reduce the size of my docker image which is using Centos 7.2
The issue is that it's 257MB which is too high...
I have followed the best practices to write Dockerfile in order to reduce the size...
Is there a way to modify the image after the build and rebuild that image to see the size reduced ?
First of all if you want to reduce an OS size, don't start with big one like CentOS, you can start with alpine which is small
Now if you are still keen on using CentOS, do the following:
docker run -d --name centos_minimal centos:7.2.1511 tail -f /dev/null
This will start a command in the background. You can then get into the container using
docker exec -it centos_minimal bash
Now start removing packages that you don't need using yum remove or yum purge. Once you are done you can commit the image
docker commit centos_minimal centos_minimal:7.2.1511_trial1
Experimental Squash Image
Another option is to use an experimental feature of the build command. In this you can have a dockerfile like below
FROM centos:7
RUN yum -y purge package1 package2 package2
Then build this file using
docker build --squash -t centos_minimal:squash .
For this you need to add "experimental": true to your /etc/docker/daemon.json and then restart the docker server
It is possible, but not at all elegant. Just like you can add software to the base image, you could also remove:
FROM centos:7
RUN yum -y update && yum clean all
RUN yum -y install new_software
RUN yum -y remove obsolete_software
Ask yourself: does your OS have to be CentOS? Then I would recommend you use the default installation and make sure your have enough disk space and memory.
If it does not need to be CentOS, you should rather start with a more minimalistic image. See the discussion here:
Which Docker base image should be used to install Apps in a container without any additional OS?
To install my microservice binaries I need a centos. And since I have 20 microservice I'm trying to find a way to optimize the images size so I'm wondering if there's a way to create a docker image without os and at the moment of deployment Docker takes the OS Layer from cache to put it in all the images.. I'm a beginner so I don't know if I'm clear in my statements ?
Yes, look at the scratch keyword (docs):
You can use Docker’s reserved, minimal image, scratch, as a starting
point for building containers.
Also you may find useful using multi-stage builds.
An example:
FROM scratch
ADD hello /
FROM fedora
RUN yum -y update && yum clean all
RUN yum -y install nginx
I'm using docker as a "light" virtual machine. For example, when I need to do some experiments on Ubuntu and don't want to mess up the host OS, I simply run docker run -it ubuntu bash.
Generally I'm very happy with it, except that I cannot keep the changes after I exit, which means I need to rerun
apt update && apt install vim git python python3 <other_tools> && pip install flask coverage <other_libraries> && .....
every single time I start the docker container as a VM, which is very inefficient.
I've noticed this question, but it only enables me to keep some specific files from being erased, whereas I want the whole system (including but not limited to all configuration, cache and tools installed) to be retained between the life cycles of the docker container.
You must use something like
docker commit mycontainer_id myuser/myimage:12
see the doc: docker commit
and then you launch your saved image myuser/myimage:12
But you should definitely use a Dockerfile
Following the section at Making your own customised boot2docker ISO, i wrote the Dockerfile below to install the vim package:
FROM boot2docker/boot2docker
RUN apt-get update && apt-get install -y vim
RUN /make_iso.sh
CMD ["cat", "boot2docker.iso"]
Then executed these commands successfully:
docker build -t my-boot2docker-img . && docker run --rm my-boot2docker-img > boot2docker.iso
I created a virtual machine using this iso image and logged into it. I've expected the vim is now available on my shell but it was not. From the build process console logs, i saw the vim installed successfully. However it is apparently not included in the iso.
Can someone please tell me, what i've missed here?
You only installed vim in the build container that produces the final boot2docker iso. To get the desired result you need to install any packages/data at $ROOTFS in the build container. For some hints on how to accomplish this with apt-get see this answer.
But first you should ask yourself why you need vim in a VM that is only meant as a transparent proxy for mac/windows users.
Edit:
As you got valid reasons to build your own boot2docker iso, have a look at the boot2docker repo.
The dockerfile broken down:
install build dependencies in the build container
download and compile a linux kernel with aufs support, copy to $ROOTFS
download and extract TinyCore distribution at $ROOTFS
download and extract TinyCore packages defined in $TCZ_DEPS to $ROOTFS
build and install VMware tools and other helpers at $ROOTFS
export $ROOTFS as new iso
I'd probably look into extending on step 4 first, i.e. simply download packages from the TinyCore repo.
I'm relatively new to Docker.
I have launch a boot2docker host using docker-machine create -d.
Managed to connect to it, and run few commands. All good.
However, when trying to create a basic http server image, based on centos..
"yum install" simply fails. No matter what is the package.
This is my Docker file:
FROM centos
MAINTAINER Amir
#Install Apache
RUN yum install httpd
When running:
docker build .
It's starting to build the image, and everything looks good.. but then fails with:
Your transaction was saved, rerun it with:
yum load-transaction /tmp/yum_save_tx.2015-09-18.15-10.q5ss8m.yumtx
SECURITY WARNING: You are building a Docker image from Windows against a non-Windows Docker host. All files and directories added to build context will have '-rwxr-xr-x' permissions. It is recommended to double check and reset permissions for sensitive files and directories.
The command '/bin/sh -c yum install httpd' returned a non-zero code: 1
Any idea what am I doing wrong?
Thanks in advance.
If you look bit earlier than the last message, you have a good chance to see something like this:
Total download size: 24 M
Installed size: 32 M
Is this ok [y/d/N]: Exiting on user command
Your transaction was saved, rerun it with:
which means you have to change the default choice, e.g.
#Install Apache
RUN yum install -y httpd