description: "Using a GitHub Actions workflow, self-hosted runners, rootless Docker, Packer, and Vault to automatically build VM templates on Proxmox."
I recently shared how I [set up Packer to build Proxmox templates](/building-proxmox-templates-packer/) in my homelab. That post covered storing (and retrieving) environment-specific values in Vault, the `cloud-init` configuration for defining the installation parameters, the various post-install scripts for further customizing and hardening the template, and the Packer template files that tie it all together. By the end of the post, I was able to simply run `./build.sh ubuntu2204` to kick the build of a new Ubuntu 22.04 template without having to do any other interaction with the process.
That's pretty cool, but *The Dream* is to not have to do anything at all. So that's what this post is about: setting up a self-hosted GitHub Actions Runner to perform the build and a GitHub Actions workflow to trigger it.
When a GitHub Actions workflow fires, it schedules the job(s) to run on GitHub's own infrastructure. That's easy and convenient, but can make things tricky when you need a workflow to interact with on-prem infrastructure. I've worked around that in the past by [configuring the runner to connect to my tailnet](/gemini-capsule-gempost-github-actions/#publish-github-actions), but given the amount of data that will need to be transferred during the Packer build I decided that a [self-hosted runner](https://docs.github.com/en/actions/hosting-your-own-runners/managing-self-hosted-runners/about-self-hosted-runners) would be a better solution.
I wanted my runner to execute the build inside of a Docker container for better control of the environment, and I wanted that container to run [without elevated permissions (rootless)](https://docs.docker.com/engine/security/rootless/).
GitHub [strongly recommends](https://docs.github.com/en/actions/hosting-your-own-runners/managing-self-hosted-runners/about-self-hosted-runners#self-hosted-runner-security) that you only use self-hosted runners with **private** repositories. You don't want a misconfigured workflow to allow a pull request submitted from a fork to run potentially-malicious code on your system(s).
So while I have a [public repo](https://github.com/jbowdre/packer-proxmox-templates/) to share my Packer work, my runner environment is attached to an otherwise-identical private repo. I'd recommend following a similar setup.
I start by cloning a fresh Ubuntu 22.04 VM off of my new template. After doing the basic initial setup (setting the hostname and IP, connecting it Tailscale, and so on), I create a user account for the runner to use. That account will need sudo privileges during the initial setup, but those will be revoked later on. I also set a password for the account.
I then install the `systemd-container` package so that I can use [`machinectl`](https://www.man7.org/linux/man-pages/man1/machinectl.1.html) to log in as the new user (since [`sudo su` won't work for the rootless setup](https://docs.docker.com/engine/security/rootless/#unable-to-install-with-systemd-when-systemd-is-present-on-the-system)).
Now it's time for the rootless setup, which starts by disabling the existing Docker service and socket and then running the `dockerd-rootless-setuptool.sh` script:
Next, I enable and start the service in the user context, and I enable "linger" for the `github` user so that its systemd instance can continue to function even while the user is not logged in:
I know I've been talking about a singular runner, but I'm actually setting up multiple instances of the runner on the same host to allow running jobs in parallel. I could probably support four simultaneous builds in my homelab but I'll start with just two runners for now (after all, I only have two build flavors so far anyway).
- Go to my private GitHub repo, navigate to **Settings > Actions > Runners**, and click the big friendly **New self-hosted runner** button at the top-right of the page. All I really need from that is the token which appears in the **Configure** section. Once I have that token, I...
- Run the configuration script, accepting the defaults for every prompt *except* for the runner name, which must be unique within the repository (so `runner1`, `runner2`, so on):
My solution for this consists of a Github Actions workflow which calls a custom action to spawn a Docker container and do the work. Let's start with the innermost component (the Docker image) and work out from there.
I'm using a customized Docker image consisting of Packer and associated tools with the addition of the [wrapper script](/building-proxmox-templates-packer/#wrapper-script) that I used for local builds. That image will be integrated with a custom action called `packerbuild`.
I don't want to maintain two copies of the `build.sh` script, so I move it into this new folder and create a symlink to it back at the top of the repo:
That way I can easily load the script into the Docker image while also having it available for running on-demand local builds as needed.
And as a quick reminder, that `build.sh` script accepts a single argument to specify what build to produce and then fires off the appropriate Packer commands:
It starts with a minimal `alpine` base image and installs a few common packages (and `xorriso` to support the creation of ISO images). It then downloads the indicated version of the Packer installer and extracts it to `/bin/`. Finally it copies the `build.sh` script into the image and sets it as the `ENTRYPOINT`.
As you can see, the action expects (nay, requires!) a `build-flavor` input to line up with `build.sh`'s expected parameter. The action will run in Docker using the image defined in the local `Dockerfile`, and will pass `${{ inputs.build-flavor }}` as the sole argument to that image.
The workflow is defined in `.github/workflows/build.yml`. It starts simply enough with a name and an explanation of when the workflow should be executed.
`workflow_dispatch` sets it so I can manually execute the workflow from the GitHub Actions UI (for testing / as a treat), and the `cron` schedule configures the workflow to run automatically every Monday at 8:00 AM (UTC).
Rather than rely on an environment file (ew), I'm securely storing the `VAULT_ADDR` and `VAULT_TOKEN` values in GitHub [repository secrets](https://docs.github.com/en/actions/security-guides/using-secrets-in-github-actions). So I introduce those values into the workflow like so:
When I did the [Vault setup](/building-proxmox-templates-packer/#vault-configuration), I created the token with a `period` of `336` hours; that means that the token will only remain valid as long as it gets renewed at least once every two weeks. So I start the `jobs:` block with a simple call to [Vault's REST API](https://developer.hashicorp.com/vault/api-docs/auth/token#renew-a-token-self) to renew the token before each run:
--request POST "${VAULT_ADDR}v1/auth/token/renew-self" | grep -q auth
```
Assuming that token is renewed successfully, the Build job uses a [matrix strategy](https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#jobsjob_idstrategymatrixinclude) to enumerate the `build-flavor`s that will need to be built. All of the following steps will be repeated for each flavor.
And the first step is to simply check out the GitHub repo so that the runner has all the latest code.
To get the runner to interact with the rootless Docker setup we'll need to export the `DOCKER_HOST` variable and point it to the Docker socket registered by the user... which first means obtaining the UID of that user and echoing it to the special `$GITHUB_OUTPUT` variable so it can be passed to the next step:
And now, finally, for the actual build. The `Build template` step calls the `.github/actions/packerbuild` custom action, sets the `DOCKER_HOST` value to the location of `docker.sock` (using the UID obtained earlier) so the runner will know how to interact with rootless Docker, and passes along the `build-flavor` from the matrix to influence which template will be created.
If it fails for some reason, the `Retry on failure` step will try again, just in case it was a transient glitch like a network error or a hung process.
All that's left at this point is to `git commit` and `git push` this to my *private* repo. I can then visit the repo on the web, go to the **Actions** tab, select the new **Build VM Templates** workflow on the left, and click the **Run workflow** button. That fires off the build, and I can check back a few minutes later to confirm that it completed successfully:
For future builds, I don't have to actually do anything at all. GitHub will automatically trigger this workflow every Monday morning so my templates will never be more than a week out-of-date. Pretty slick, right?
You can check out my *public* repo at [github.com/jbowdre/packer-proxmox-templates/](https://github.com/jbowdre/packer-proxmox-templates/) to explore the full setup - and to follow along as I add support for additional OS flavors.
