build: support local state group

[crazy-max]

follow-up #1735

We currently support saving local state for a build ref but we can't associate multiple refs when building a group of targets. This PR adds support for saving the local state group so we can identify refs that belong to the same group of targets.

[crazy-max]

Reworked a bit build invocation in first commit as BuildWithResultHandler had a bunch of args already so we can now pass variadic opts when instantiating Build instead. Wanted this as I was thinking passing a localstateHandleFunc but would need #1966 first as resolving the node of the build before saving its state would be necessary but seems enough to just pass localstate.StateGroup object instead.

Now it saves the raw definition, inputs and targets for the group. Also needs to save the target for each state ref so we can associate them with the definition.

Didn't set yet hcl variables but I guess we need a new Variables field to bake.Config. It would be ok for HCL definitions but would not have any meaning for compose files.

[crazy-max]

@tonistiigi As discussed, remove variadic opts and adds another arg to build.Build. Also needs to set build ref(s) early as well which requires a new Ref field to build options.

[tonistiigi]

So no variables? Ok by me to leave this for follow-up but comments seem to suggest that this was already added.

[crazy-max]

Will look in follow-up. It's a bit tricky as the matrix feature hacks around variables so we can't just pick them.

[jedevc]

I think if you want the values that the variables are evaluated to, you'd need to modify hclparser.Parse to return that information. I don't think there's a way to extract the vars/funcs defined in the HCL today 😢

Apologies, the code around there is a total mess - happy to help out 😄

[crazy-max]

you'd need to modify hclparser.Parse to return that information.

Yes indeed, I have started to look at it and how I could use context evaluation to extract vanilla variables. Will open a PR and let you know, thx!