docs(hetzner): update for new OEM images

[apricote]

Update Hetzner docs for new OEM images, explaining the Snapshot process

Now that the beta and alpha releases include Hetzner OEM images we can update the docs to utilize these images.

The current terraform process is okay for a single server, but if you want to create a fleet of servers you want to re-use the snapshots to create new servers quicker.

I was not sure which option is actually easier, Packer or hcloud-upload-image, so I documented both for now.

In comparison to https://github.com/apricote/flatcar-packer-hcloud/ I tried to remove any bells and whistles from the Packer template, to keep it as short as possible for the docs and easy to understand.

Related to flatcar/Flatcar#1412.

Disclaimer: I am the author of hcloud-upload-image and the mentioned "extended packer templates". I also work at Hetzner, but this was done in my private time.

How to use

If you have a Hetzner account, you can just follow along with the instructions.

Testing done

I have followed these steps myself while writing the docs to verify that they work.

• Changelog entries added in the respective changelog/ directory (user-facing change, bug fix, security fix, update)
• Inspected CI output for image differences: /boot and /usr size, packages, list files for any missing binaries, kernel modules, config files, kernel modules, etc.

[tormath1]

LGTM. There are two small comments to address and I think we can add a note regarding the Afterburn issue you found directly under the Butane example.

EDIT: we can squash everything in one commit.

[apricote]

Cleaned everything up, fixed the Afterburn issue and squashed it all into a single commit :)

[tormath1]

Many thanks for all your efforts and your contributions regarding Hetzner and Flatcar support. 🙏