until now, ${u} would match all users,
${u%-foo} would exclude users in group foo,
${u%+foo} would only include users in group foo
now, the following is also possible:
${u%-foo,%-bar} excludes users in group foo and/or group bar,
${u%+foo,%+bar} only includes users which are in groups foo AND bar,
${g%-foo} skips group foo (includes all others),
${g%-foo,%-bar} skips group foo and/or bar (includes all others)
see ./docs/examples/docker/idp/copyparty.conf ;
https://github.com/9001/copyparty/blob/hovudstraum/docs/examples/docker/idp/copyparty.conf
just like before, if vpath contains ${u} then
the IdP-volume is created unconditionally
but this is new:
${u%+foo} creates the vol only if user is member of group foo
${u%-foo} creates the vol if user is NOT member of group foo
* allow mounting `/` (the entire filesystem) as a volume
* not that you should (really, you shouldn't)
* improve `-v` helptext
* change IdP group symbol to @ because % is used for file inclusion
* not technically necessary but is less confusing in docs
