Add support for tmpfs mounts on Docker runtime

[runabol]

As opposed to volumes and bind mounts, a tmpfs mount is temporary, and only persisted in the host memory. When the container stops, the tmpfs mount is removed, and files written there won't be persisted.

This is useful to temporarily store sensitive files that you don't want to persist in either the host or the container writable layer.

Limitations:

• Unlike volumes and bind mounts, you can't share tmpfs mounts between containers.
• This functionality is only available if you're running Docker on Linux.

The goal of this issue is to add support for a tmpfs mount.

Rough outline of the implementation:

1. Add an tmpfs.go file to the docker packge.
2. Implement the Mounter interface.
3. Register the new mounter implementation when wiring in the Docker runtime.
4. Add tests.

[runabol]

#211