Add support for configuring additional build contexts

[Laura Brehm]

What is the problem you're trying to solve

BuildKit added the --build-context argument to enable users to provide additional build contexts during a build. More documentation about this can be found here.

Describe the solution you'd like

Being able to configure additional build contexts in a Compose file, such as:

services:
  a:
    build:
      context: .
      build_contexts:
        - project=[a directory/image/etc.]

Used together with a Dockerfile such as:

# syntax=docker/dockerfile:1
FROM alpine:latest
COPY --from=project . /

Additional context
Add any other context about the feature request here.

• docker/compose#9961 => microsoft/vscode-remote-release#7305

[Nicolas De loof]

We should just use contexts as this is in the build section already

[Laura Brehm]

Sounds good! I was wondering whether having contexts next to context would be confusing. I wonder if it should be extra_contexts or something similar to be more explicit that it's optional (as opposed to context). WDYT?

[Nicolas De loof]

additional_contexts or named_contexts could indeed avoid confusion.
Also would probably better use of a mapping vs the name=path syntax introduced by buildx as a command line flag doesn't have much flexibility (we could support both, as we do in a few places already)