feat(dockerfile): make the image build succesfully

- Break the system dependency install out into separate RUN commands to
  take advantage of caching while building.
- Get the Xvfb starting up correctly:
  - Remove the x11vnc dependency.
  - Add the Xorg dependency.
  - Create a machine-id for dbus.
  - Add commands for Xvfb and firefox to the start script.
- Get sshd running:
  - Create the ssh keys for the server.
  - Install openssh-server.
  - add it to the start script.
- Use the start script as a single point of entry.