Use labels instead of environment variables

[mariusGundersen]

The latest version of docker let's you label a container with metadata, associating a string value to a string key. This seems like a better fit for VIRTUAL_HOST than using environment variables

[thaJeztah]

I think both can make sense. One advantage of env is that they are accessible inside the container, which can be handy.

Nonetheless, adding support for labels could be nice :)

[md5]

I believe this can be closed now.

[jwilder]

Fixed by #82