Docker

Docker is a container virtualization environment which can establish development or runtime environments without modifying the environment of the base operating system. It has the ability to deploy instances of containers that provide a thin virtualization, using the host kernel, which makes it faster and lighter than full hardware visualization.

Kernel
Kernel version 3.10 or greater is required in order to run Docker.

If the kernel has not been configured properly before installing as list of missing kernel options will be provided. These kernel features must be enabled manually. (forward slash) can be used while in the ncurses-based menuconfig to search the name of the configuration option.

After exiting the kernel configuration, rebuild the kernel. If the kernel rebuild as also a kernel upgrade be sure to rebuild the bootloader's menu configuration, then reboot the system to the newly recompiled kernel binary.

Emerge
Install :

OpenRC
After Docker has been successfully installed, start it and add it to the system's default runlevel:

Permissions
Add relevant users to the docker group:

Testing
In order to test the installation, run the following command:

That will first download from the Docker Hub the image named hello-world (if it has not been downloaded locally yet), then it will run it inside new namespaces. It purpose is just to display some text through a container.

Building from a Dockerfile
Create a new Dockerfile in an empty directory with the following content:

Run:

Own images
There are two different ideas how a container should be built.
 * The minimal approach: According to the container philosophy a container should only contain what is needed to serve one process. In this case ideally the container consists of one static binary.
 * The VM approach: A container can be treated like a full system virtualization environment. In this case the container includes a whole operating system.

Build environment for the image
The image can be created out of a live system or - preferably - out of a special build environment
 * To create a build environment for the image, follow the Cross_build_environment guide. There is no need to emerge a full @system. The build essentials are enough.
 * The toolchain tuple could look like
 * The build essentials can be build like this

The minimal approach: Statically linked binaries
There are some caveats with this. The hints for statically linked binaries should be kept in mind for this. To build an nginx-image
 * Chroot into the build environment (e.g. )
 * Build the desired package statically linked
 * Extract the binary package to a tmp dir (e.g. )
 * Change the nginx configuration. At least add  and swap   for.
 * Add, ,  and a appropriate  directory. Make sure the   has "files" instead of "compat" and your   has an "nginx" user entry.
 * Create the docker image out of the current directory
 * Spawn a container and start nginx

The VM-like approach

 * Create the image out of the full environment
 * Spawn a new gentoo container and start a shell:
 * This image can used as a base image. To build a nginx image for example run emerge nginx inside the container and push it back as new image afterwards:

External resources

 * https://docs.docker.com/installation/gentoolinux/ - Official Docker installation documentation.