tools: Add a Dockerfile for building docker containers

For now these build WIP builds rather than a specific version
of Cockpit.

To run in a container:

 * We use SSH to log back into the host
 * We use the host net namespace to be able to:
   - Listen on port 9090
   - Connect to 127.0.0.1 on the host
   - Connect to the system journal
 * Copy the cockpit-bridge and dependencies into the host
   at /opt/cockpit for the user session
 * Drop necessary files in /usr/local on host to make it run

Closes #1446
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>