MERGE statement for targets

[hidmic]

Use case

Spiritually related to #2480. Earthly is amazing as it is but building interdependent software packages is still somewhat painful. To the best of my knowledge, when you have a complex target DAG you are forced to either:

1. linearize your DAG, which sacrifices modularity and breaks down when the thing doesn't look like a tree
2. craft COPY statements to pull the necessary artifacts from each dependency, which is brittle when you don't have total control over the layer content
3. have a function that implements each target so you can chain them freely through invocation, which requires some care with build contexts (and still gets messy when function+target pairs are split across local and remote Earthfiles using COPYs)

Even if functions were meant to be the escape hatch, I believe first-class support for complex target DAGs would be a great feature. And since Buildkit supports merge and diff operations already, I imagine a MERGE statement for targets wouldn't be far fetched.

Note: In all honesty, this isn't as much of a proposal as it is a question. I see COPYs are merged copies (à la COPY --link), so I imagine this crossed your mind already and for some reason you discarded it. I wonder why.

Expected Behavior

1. Given the following Earthfile:

   VERSION 0.8
   
   foo:
       RUN apt-get update && apt-get install -y foo
   
   bar:
       RUN apt-get update && apt-get install -y bar
   
   baz:
       MERGE +foo +bar
       SAVE IMAGE baz:latest

2. Run earthly +baz.

3. Confirm that baz:latest history shows both foo and bar.

MERGE could be a single llb.Merge operation or recursively expand to compute merge-diffs by interleaving llb.Diff and llb.Merge operations.

[hidmic]

Hmm, looks like this didn't draw much attention 😅