Summary:
Defined new abstract `build.Volume` and corresponding
`docker.DockerVolume` instructions.

Refactored compilation of main `COPY` or `VOLUME` instruction for
application files to use the new instructions and moved injection of
these instructions out of the compiler and into `VariantConfig`. The
latter can be smarter about the following cases:

 1. When `copies` is set, simply depend on artifacts for the application
    files and do not copy anything from the build host.
 2. When `sharedvolume` is `true`, inject a `build.Volume` instruction
    for the application working directory.
 3. When neither of the above are set, copy application files from the
    host.

Fixes T174623
Depends on D768

Test Plan:
Run `go test ./...`. Run `blubber blubber.example.yaml production` and ensure:

 1. The `prep` stage has a `COPY . .` instruction.
 2. The final stage has no `COPY . .` instruction, only `COPY --from=prep`
    instructions.

Reviewers: thcipriani, mobrovac, hashar, mmodell, #release-engineering-team

Reviewed By: thcipriani, mobrovac, #release-engineering-team

Tags: #release-engineering-team

Maniphest Tasks: T174623

Differential Revision: https://phabricator.wikimedia.org/D769