Proposal: Docker Machine Integration

[dave-tucker]

Following on from the discussion in #19, here is a mock up of what integration with Docker Machine could look like.

The player gets dropped in to a lobby when connecting:

On each of the portals is a sign that gives the name of the "Machine" it represents.
Stepping through the portal will teleport the player to the world where they can interact with that machine, much like the default Dockercraft world today... The only difference being that we have a portal that will take us "back" to the lobby world.

We'll support two modes of use:

1. When you bind mount the docker socket, we will create a "default" machine and use the mounted socket to communicate with the Docker API. This is for backwards compatibility.
2. Instead of mounting the one socket, you can mount your entire .docker folder which will create a portal for each of your machines
   Portals are added in a straight line, first to the left of the existing portals, then to the right. This means that the "default" portal will always be dead centre and that the line should extend evenly in each direction.

Under the hood, we use libmachine to query the list of created machines and to get the necessary certificates to set up a secure API connection over HTTPS.