Use a volume mount by default to run everything in docker

[jameshalsall]

TODO:

• [x] settle on the command API
• [x] update the docs
• [x] test :)
• [x] only set -T on docker-compose exec commands when needed (pipelines)
• [x] fix the ws go test integration <test-name> command
• [x] ensure pipeline integration tests run OK

https://github.com/inviqa/go-sample/pull/16 is a demonstration of this working in pipelines.

This would close #196.

NOTE: I've labelled this as a breaking change, but it shouldn't break any workflows, it should just make them more efficient. The main thing that will change is the ws recompile command, which will no longer rebuild the whole app container.