Refactor images lists and default mounts into base templates

[jandubois]

Also adds template://default/mounts to setup the default mounts globally.

Closes #2418

If we decide to do this, then we should also create images/* templates for the other distributions.

[jandubois]

I wonder how we should deal with templates/default.yaml. Ideally it should also use the base templates, but then we are missing out on the comments.

Maybe include some commented out sample values in the default template, but still use the base templates to get the images and mounts?

[jandubois]

The tests also use the templates via filename locator (templates/docker.yaml) without installing the templates first.

Which means all the references to base templates should be relative locators: _images/ubuntu-lts.yaml instead of template://_images/ubuntu-lts. I've changed this PR to accommodate.

Which means the templates in templates/experimental cannot use the new base templates because ../_images/ubuntu-lts.yaml is invalid; all relative references must be in or below the directory of the parent template.

So I think I should revert the change and reference the base templates via template:// again. Which means that we either need to install the templates before we need use them, or we could add a LIMA_TEMPLATES variable so we can override the location of the template store.

I would prefer to add LIMA_TEMPLATES.

[AkihiroSuda]

LIMA_TEMPLATES

Maybe LIMA_TEMPLATES_PATH, with the support for PATH separators (:)

[jandubois]

Maybe LIMA_TEMPLATES_PATH, with the support for PATH separators (:)

I'll take a look, but this is now getting into scope-creep for a quick PR that I hoped to do on the side while working on something else. 😄

[jandubois]

This turned into a bit bigger PR than originally expected. I think it is getting close now, so please do another review.

We could move other distro images to _images/* as well, but since they are not currently duplicated, it would not really simplify anything. It would just make it easier to reuse the image lists from end-user templates.

That may be enough of a reason to do it, but maybe it should be done at a later point in time, when we have some experience/feedback on the current refactoring?

[jandubois]

@AkihiroSuda PTAL! I hope CI will be green (except for the unrelated Windows failure). I will not be making any more changes today, and I hope this is close to "done" now.

Hope I don't jinx it by marking it "Ready for review". 😄

[AkihiroSuda]

Thanks

[AkihiroSuda]

Now WARN[0000] base is experimental is printed for almost every invocation of limactl start.

Can we make it out of experimental now?