Allow overriding container restart policy

[markmetcalfe]

This adds a new .env option of RESTART_POLICY which allows overriding whether a container will restart automatically after rebooting or crashing. This is handy as it means you can make it so that you don't need to tup your containers everytime you start your computer. This is a backwards compatible change, where it will default to the status quo.

# Set the restart policy for containers
# If set to 'no', containers will only start when you manually start them via `tup`
# If set to 'unless-stopped', containers will restart upon rebooting/crashing
RESTART_POLICY=unless-stopped