Named build context with multiple sources

[SpecLad]

Description

For my use case, it would be useful if you could create a single additional build context from multiple sources.

For example, suppose I have a directory /tmp/a, containing a.txt, and a directory /tmp/b, containing b.txt. I would like to be able to run something like the following:

docker buildx build . --build-context foo=/tmp/a --build-context foo=/tmp/b

And have the build context foo contain both a.txt and b.txt.

I only need this to work when both sources are local directories, although for consistency it would be nice if other source types were supported too.

[tonistiigi]

You can solve this with a bake target that does the combining of the context and sets result as an input to other target. For build command I think this case is too app-specific.