Summary:
Introduce new `build.EntryPoint` and `build.WorkingDirectory`
instructions to allow configuration to inject them instead of hard
coding their generation in the Docker compiler.

Simplified the Docker compiler to simply iterate over build phases as
returned by a new function `build.Phases()`.

Depends on D990

Test Plan: Run `go test ./...`.

Reviewers: thcipriani, demon, hashar, #release-engineering-team

Reviewed By: thcipriani, #release-engineering-team

Tags: #release-engineering-team

Differential Revision: https://phabricator.wikimedia.org/D991

Summary:
Wrote inline documentation for all the things.

Fixes T168000

Test Plan: Execute `godoc -http=:6060` and proofread all the things.

Reviewers: thcipriani, hashar, #release-engineering-team, demon

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

Tags: #release-engineering-team

Maniphest Tasks: T168000

Differential Revision: https://phabricator.wikimedia.org/D841

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

Establish phases within Docker compiler to allow providers (apt, npm,
etc.) to inject their own run/copy instructions into to the Dockerfile
compilation process while leaving the compiler agnostic to the providers
themselves.

The instructions and phases are also generalized to leave room for
alternative compilers should they be needed in the future (e.g.  aci
support via acbuild) but also as a general design constraint to leave
compiler implementation concerns out of providers.