Nick Sellen authored Sep 28, 2017 and Matthias Larisch committed Sep 28, 2017

Now we don't run build every time in start/test some changes will
require a rebuild.

I *think* any time you update something in `docker/` other than
the docker-compose files it will need to be rebuilt.

It uses a checksum/hash type approach for seeing if the
docker-compose config files have changed.

For this reason we should consider putting more things into the base
images and just referring to the image directly in the docker-compose
config where possible.