Change the prefix of created containers to namespace between modes

[KitB]

e.g. instead of project_transcoder_1 we might have project_dev_transcoder_1

[KitB]

Knowingly broken in 0.4.0 because of redesigned mechanism, harder to do now.

[KitB]

At the moment all we can do (I think?) is set an environment variable. This will mean the user has to run compose at least once in each terminal. It looks like there are provisions on the way (docker/compose#3118) for putting this into docker-compose.yml so maybe we should just wait until then.