Use Alpine images for cached image test

[MusicDin]

Use smaller Alpine images for testing cached images.

[simondeziel]

While in general I'm happy with smaller images, it doesn't seem to have a great influence on CI run times. Ignoring the fact they currently fail for a reason trivial to fix, the tests with the Ubuntu image would take ~11 minutes as well.

Is the speedup more noticeable locally?

[MusicDin]

I'm not sure why, but using ubuntu remote it occasionally takes >5min to copy an image.

For example https://github.com/terraform-lxd/terraform-provider-lxd/actions/runs/9345944914 took ~30 minutes for cached images.

For local tests it is definitely a speed up. If we want to keep ubuntu remote, I would switch the images used for cached image tests and other tests (as ubuntu image will be cached and not re-downloaded multiple times during the tests).

[simondeziel]

Agreed, ubuntu: and other CPC maintained remotes were terribly slow (like ~500kbps) over the weekend.