Handle error output of docker commands

[hakkanicko]

Rather than mix stdout and stderr of docker commands, I suggest a way to handle stderr separately.

See #187

[palantirtech]

Thanks for your interest in palantir/docker-compose-rule, @hakkanicko! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

[sirlatrom]

@hakkanicko Do you think you'll be able to move on and sign the agreement so others can gain from your contribution? 🙂