Containerize integration tests and development

Scripts in [integration/](https://github.com/google/trillian/tree/5abce90b597eaadd29cdbf83042f9ec9255b7f7c/integration) are hard to maintain, and contain bugs. For example, after running `run_integration.sh` there are dangling Trillian servers.
Also, a developer needs to install tooling that exactly corresponds to Trillian CI (MySQL, Go, etc), which is sometimes not easy to do.

Running Trillian locally is much easier using Docker[-compose]. Most CI providers support containers anyway. We can make it possible to run Trillian locally in exactly the same state as in CI, using containers.

https://www.docker.com/blog/containerize-your-go-developer-environment-part-1/