Integration tests should use Docker image

There should be a run of the integration tests using Trillian and the CTFE running from within Docker containers. This would check that the Docker configs are working correctly and that the CTFE is compatible with the latest published images for Trillian. This could be extended to also try the latest tagged release of Trillian, to detect changes in CTFE that are not backwards-compatible.

Trillian's integration tests already use its Docker images, see:
https://github.com/google/trillian/blob/72c17b947c009a93a424a734829f113094419986/.travis.yml#L112