* guix/scripts/environment.scm (launch-environment/container): Add UID
and GID. Use them in PASSWD and GROUPS. Pass them as #:guest-uid and
#:guest-gid to 'call-with-container'.
* tests/guix-environment-container.sh: Test the inner UID.
In '--user' test, replace hard-coded 0 with 1000.
* doc/guix.texi (Invoking guix environment): Adjust accordingly.