Improve convenience for users running BuildStream through `docker`

BuildStream has limited support for non-Linux platforms, and in particular it doesn't work natively on Mac OS X. We have users on that platform who are using docker to provide a minimal Linux environment capable of running BuildStream.

The process is documented here: https://buildstream.gitlab.io/buildstream/install.html#using-buildstream-inside-docker

However:

• it's presented last
• it tells users to build the Dockerfile themselves, when they could just use the official images we provide
• it gives instructions on running the test suite before any instructions on how to just use BuildStream
• it requires the user to copy and paste 4 lines of shell in order to run bst

We should improve this:

• recommend the official Docker images; move "how to build" instructions somewhere else
• add a docker-bst wrapper script to automatically run BuildStream inside a container
• make sure we test BuildStream in Docker ourselves during CI, don't expect users to do that for us