I give up now after numerous attempts to reduce the docker image size.
The only thing left is that I've put the NODE_ENV=.. configuration in
the docker-compose.X.yml files, I think that's slightly cleaner
@mattwr18 ouch, that was a hard to spot error. It struck my eye when I
was switching back and forth between `docker-compose.override.yml` and
`docker-compose.travis.yml`.