Dockerfile: Fix bug by adding group by id not name

[sjrct]

I was running into this issue on solus trying to build the image with a specified gid when the gid (15) already existed.

When a group already exists with the specified gid, usermod -f will silently added the group with a default new gid (eg 1001). So worker ends up with an extra useless group rather than the one specified. This fixes the issue by adding the group by the gid rather than by the name, so worker always ends up with the gid you want (even if that isn't the newly created optical group)