This is achieved with these techniques:
- Use Alpine as the base image, which is only ~5MB.
- Use docker's multi-stage build to keep only the result binaries in
the final image.
The new image is ~15MB after this change.
Also updated Dockerfile to use the current checkout code instead of always
sync from the latest.
Also added a .dockerignore file to ignore temporary build artifacts.
Closes#535.
Change-Id: I3c90805ebba40295e69241214ed6d7adbde465b8
