Docker support

[pataquets]

To be able to build Barnard with Docker.
Additionally, it could be distributed as a public Docker image to avoid installing Golang build deps.

[pataquets]

@layeh: Ping, in case you didn't get notifications.
I can help to create a regular, runnable Docker image or just a builder container as I did here.

[pataquets]

@bontibon ping, in case you're not getting notifications.