Switch to shairport-sync user when running shairport-sync

[jonathandann]

The user and group shairport-sync will be created if necessary, and a runtime folder will be created at /var/run/shairport-sync [...] - https://github.com/mikebrady/shairport-sync

To Fix

1. See what user the regular install of shairport is running as
2. Verify the install script in the container is creating this user
3. USER $username to switch https://stackoverflow.com/a/24555761

[jonathandann]

If the container is running avahi as part of /start.sh then using the USER command in Dockerfile might not be the right solution.