Make obfs4 Docker image more usable

Here is some feedback we got from an operator (see this blog post for the full story):

• Make it easier to get the bridge's fingerprint and/or bridge line. At the moment, users have to spawn a shell in the container, which is tedious.
• Maybe provide a docker-compose file.
• Improve our official setup instructions. These instructions were more helpful to an operator.
• Add a note that operators can run docker logs <container> to check if it's up and running.
• Mention concerns regarding permanence: Ideally, a container should run as long as possible.
• Allow running a bridge on a port <1024 (as per mrphs's request in comment:2).