[Refactor] Sources with subpaths are not optimal

[cpuguy83]

Expected Behavior

No response

Actual Behavior

When we have a Source with a subpath added we are, in most cases, performing a copy to shift the root of the state to that subpath.

E.g. if we have a source:

sources:
  src:
    path: some/path
    context: {}

We first fetch the source, e.g. llb.Local("context", ...) and then we have to llb.Scratch().File(llb.Copy(st, "some/path")).

However, we don't really need to shift the root of these states like this if we provide a higher level abstraction than just an llb.State.
We would be able to, in most cases, use a mount with a subpath which is much cheaper and plumb that through to all the various parts that want the state.

Steps To Reproduce

No response

Are you willing to submit PRs to contribute to this bug fix?

• Yes, I am willing to implement it.