I work with Linux containers and container images everyday. I often create new images using podman or buildah. One day I needed to build a custom image of a virtual machine and in this post you'll read how I did it.

Prerequisite

In the post GitLab custom executor for Openstack I explained how to create a GitLab executor for Openstack. That executor provisions an Openstack instance for each job and run a job script in it. There is some required software to run jobs:

The user must set up the environment, including the following that must be present in the PATH:

Git: Used to clone the repositories.

Git LFS: Pulls any LFS objects that might be in the repository.

GitLab Runner: Used to download/update artifacts and cache.

It means that we need to customize an instance image to include prerequisites.

libguestfs

After some research I found a tool that does exactly what I need. libguestfs project provides a set of utilities for accessing and modifying virtual machine disk images. virt-customize can customize disk images in place. I chose Fedora Cloud image as environment for running GitLab jobs. The commands to add required software are very simple:

1. Install libguestfs
2. Download or build gitlab-runner

3. Create a file with a virt-customize scenario:

   sh cat > commands.txt <<EOF install git-core,git-lfs copy-in <path to gitlab-runner binary>:/usr/bin/ chmod 0755:/usr/bin/gitlab-runner selinux-relabel EOF

4. Run virt-customize:

   sh virt-customize -v -a Fedora-Cloud-Base-35-1.2.x86_64.qcow2 --commands-from-file commands.txt

Pay attention to the selinux-relabel customization option. This is required for VMs that supports SELinux. Fedora instance cannot properly start if you modified the image without relabelling.

References:

• https://libguestfs.org/virt-customize.1.html
• https://docs.gitlab.com/runner/executors/custom.html#prerequisite-software-for-running-a-job
• https://alt.fedoraproject.org/cloud/

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