Summary:
Refactored the Docker compiler to omit the stage name (`AS <stage>`)
from the `FROM` instruction when no dependent artifacts are declared.
This allows for use with older versions of Docker that don't support
multi-stage builds.

Test Plan: Run `go test` under the `docker` directory.

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

Reviewed By: mobrovac

Tags: #release-engineering-team

Differential Revision: https://phabricator.wikimedia.org/D684

Summary:
Refactored bool config fields to use a new `config.Flag` type that keeps
track of whether it was set by unmarshalled data or merged from another
instance, fixing the behavior of these fields when overwritten by
variants.

Fixes T166353

Test Plan: Run config unit tests (`cd config; go test`)

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

Reviewed By: mobrovac

Tags: #release-engineering-team

Maniphest Tasks: T166353

Differential Revision: https://phabricator.wikimedia.org/D680

Summary:
  Added a Makefile so you don't need any special mangling of your go path

Test Plan:
  None

Reviewers: dduvall, mmodell, #release-engineering-team

Reviewed By: mmodell

Subscribers: Joe

Tags: #release-engineering-team

Revert Plan:
  None

Differential Revision: https://phabricator.wikimedia.org/D671

Summary:
Now that the project is hosted on Phabricator the import URIs must be
changed.

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

Reviewed By: #release-engineering-team, demon

Tags: #release-engineering-team

Differential Revision: https://phabricator.wikimedia.org/D657

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.

It sounds more declarative.

Reorganized config type declarations.

Added `RunConfig` for declaring working directory and unprivileged
runtime user. A system for enforcing restrictions should probably be
implemented for this.

Implemented Dockerfile compiler, supporting multi-stage output in cases
whether artifacts from other variants are configured.