Image should support pulling artifacts from docker images to build the context

[jlewi]

• Redo Image API
  • Refer to docs/image_build.md
  • Image should support pulling artifacts from docker images to build the context
• The "base" for sourceMappings should be specified as a URI
  • URI could be a docker image or a git repo
  • src locations are relative to that resource
• The hydros CLI will take care of replacing the location of the git repo with the local cloned copy if there is one.
• Allow the location of the Dockerfile in the context to be specified