Edit Jun26: Added support for umask, created separte USER_ID and GROUP_ID, and split out the remapping function into a new mapids.sh utility
When containers write files to volumes shared with the host, we see several problems:
Apps run as the "wrong" user, creating files with ownership other than nobody:users
Apps run commands like "chown -R nobody:users", but the UID for nobody and the GID for users may not match the host
The container's default umask might be different, causing fi