Shrink docker image.
Update from fig (deprecated) to docker-compose.
Update README.md.

Signed-off-by: Tiago Pires <tandrepires@gmail.com>

The previous setting would cause all repo creations to fail, as
described in issue #328.
The previous commit also resolves this issue, but it seems saner to
create the user in the Dockerfile than at runtime.

Why not use the environment variables, when they are there...

The dev Dockerfile now works without superfluous go get calls.
At this point the -u option is really not what we want.

Rewrite of the docker setup. Now uses fig to manage containers and container
linkage. The base is a block based mix and match, which will give you the
possibility to easily test all configurations.