Rework the Dockerfile

- Have two Dockerfiles doing essentially the same,
  but optimized for build time (for tests)
  and image size (for Docker Hub)
- Fetch gx dependencies
- Expose port 4002 for utp
- Specify go version, currently 1.5.3-r0
- Create ephemeral fs-repo if none is mounted
- Have t0300-docker-image actually test IPFS, not just an echo
- Make everything a bit less hardcoded
- Remove dead shacheck

License: MIT
Signed-off-by: Lars Gierth <larsg@systemli.org>