docker-ci 0.5.6: Fully dockerize docker-ci. Add build test coverage. Add backup builder.

Docker-DCO-1.1-Signed-off-by: Daniel Mizyrycki <daniel@docker.com> (github: mzdaniel)
Merge pull request #4163 from mzdaniel/docker-ci

docker-ci 0.5.6: Fully dockerize docker-ci. Add build test coverage. Add backup builder.