include source repo, revision and go package version as build-args

[deitch]

- What I did

Added some fixed default build-args that linuxkit pkg build passes to the docker build. These are

• SOURCE
• REVISION
• GOPKGVERSION

They are in the docs as well.

These are set only if the user does not override them in build.yaml.

Also added a test, of course.

Note that I picked build-args that are tied to the filesystem and commit. Thus, they do not interfere with reproducible builds.

I did consider adding the username and host that built, but wasn't sure if that violated our "reproducible always" requirement.

- How I did it

Changes to git.go and build.go

- How to verify it

CI. I added tests. And I tested manually.

- Description for the changelog

Pass build-context information to package builds via --builg-arg

- Note

I have a follow on for several packages to take advantage of this.