Run some services as non-root users

This is a partial implementation to fix #26 (closed), the following services now run as a non-root user:

• salad
• vpdu
• local-registry
• fdo-proxy-registry
• gitlab-runner

These services still run as root:

• telegraf -- this runs as a privileged container... why?
• executor -- runs dnsmasq, which needs to bind to ports <1000