Make gentoo-based docker image

As a gentoo user, I want to build gentoo-based docker image to can deploy to other Linux distributions. Such a docker image is usually faster when compared to another one generated otherwise. This post documents my experience. In general, there are several steps as shown below.

1. Prepare systemd-based stage3 image (mine is stage3-amd64-systemd-20190323.tar.bz2)
2. make a chrooted environment, as we do in standard gentoo installation.
3. Install the package into an initially empty directory.
4. Make docker image from that installation directory.

Step 3 is the key. To do so, we need to use ROOT environment variable during emerge. Suppose that we want to make an image running nginx service. The command is as follows.

ROOT=/mnt/nginx-docker emerge bash shadow glibc nginx

(Tips: we can futher chroot to /mnt/nginx-docker to check if we can launch the service.)

The reason why emerging bash/shadow/glibc is that we use dynamic shared libraries when building nginx. Although I didn't try, I suspect that use ```emerge -e nginx``` would be OK in this case.

After emerge finishes, exit the chroot environment and run the following command:

cd /mnt/gentoo/mnt/nginx-docker tar --numeric-owner -cj --to-stdout . | docker import - gentoo-nginx:base

What follows is standard docker image making.

Although it is fun to do so, I would not recommend other people to make gentoo-based docker images because there are many dark corners out there. If I really need a gentoo-based docker image, I would pull the gentoo base image from docker hub, emerge packages, and make my docker image.