Publish built images in sysext-bakery with GitHub Action as GitHub releases for systemd-sysupdate

[pothos]

Current situation

We have some build scripts for systemd-sysext images but this still is not user friendly because requires the user to build and host themselves if they want to use systemd-sysupdate for live updates.

Impact

Complicated

Ideal future situation

We have GitHub Actions that publish releases with the built images as attached artifacts and the SHA256SUMS systemd-sysupdate manifest file that lists all artifacts with name and checksum, and also example configurations for how to set it up with systemd-sysupdate.
This way users can specify the sysupdate config as URL in their ignition file (or they copy the config over). They can also specify the images for first boot in Ignition by URL or let systemd-sysupdate do the download (this requires upstream support to run systemd-sysupdate from the initrd, or a workaround by specifying to run systemd-sysupdate in userspace before the sysext service starts up).

The sysupdate configuration should use the /latest/ URL to resolve to the latest GitHub Release's manifest file. Each systemd-sysext image should contain a version suffix. The sysupdate config should use use the current-symlink setting to point to the currently enabled version.

Implementation options

I think it should be ok for systemd-sysupdate if we only include the latest release in the manifest file as it will see the old one on disk.

Additional information

No additional work for the build scripts should be needed, they are expected to use upstream binaries where possible and only be compiled if required.

[tormath1]

I'm interested to have a look at this one. (FYI @mdbooth, that could help to host Kubernetes sysext image and then update CAPO templates to consume this image - as discussed on Slack)

[mdbooth]

Perhaps something to discuss in CAPO office hours: (c|w)ould we publish experimental cluster templates?

[tormath1]

Build and release of the images is done in flatcar/sysext-bakery#16 - some documentation is missing and why not generate sysupdate configuration for each component as suggested by @pothos

[pothos]

Finished in flatcar/sysext-bakery#22