Allow usage with --user option

[devster31]

This is a feature request to have this container work with --user [docker] and user: [docker-compose] options.
The way the current startup sequence is structured requires running chown to a container user with a predefinite UID and GID.
Other images, however, (e.g. official postgres here for example), use lines like chown -R "$(id -u)" "$PGDATA" 2>/dev/null || :.
Testing for $(id -u) = 0 and changing znc:znc to "$(id -u):$(id -g)" in:

znc-docker/slim/50-chown.sh

Line 4 in 207358a

chown -R znc:znc "$DATADIR" || exit 1

as well as tweaking su-exec from

znc-docker/slim/99-launch.sh

Line 5 in 207358a

exec /sbin/tini -- su-exec znc:znc /opt/znc/bin/znc --foreground --datadir "$DATADIR" "$@"

should be sufficient to allow arbitrary --user values and not break anything.

[DarthGandalf]

Thanks for the advice!