remove the tmux setup from android-dev image

Probably better to put non-trivial setup like that in a child
container.

If `/start_dev_env.sh` is present, e.g. added in child, it is
executed and passed $DISPLAY as its first argument.  Otherwise,
a urxvt terminal is started.
give dev user sudo access
better comments in start.sh
extra dev packages like git belong in a child container

Or they can be installed interactively.  For example with:

  sudo pacman -S git

after attaching to the terminal with xpra.
chown dev user's home directory

Fix permissions on startup in case child container screws 'em up.
One way that can happen is if the Dockerfile clones a git repo
into /home/dev as root.
Add Dockerfile for working with MAXS project

See:
  http://projectmaxs.org/
  http://projectmaxs.org/documentation/developer_quickstart.html