Allow JEKYLL_UID/JEKYLL_GID with existing IDs

Some standard UIDs/GIDs already exist in the docker image, e.g.
nobody, nogroup, users. If JEKYLL_UID/JEKYLL_GID is used with a numeric
ID matching one of those, then entrypoint aborts with an error like
"groupmod: GID '33' already exists".

This commit adds the -o option to usermod/groupmod to allow non-unique
IDs.