Fix building base image on Mac. (#22)

Mac uses low group IDs for the default user, which would cause groupadd
to fail as the group already existed. Only make a new group if it
doesn't already exist.

This does raise other questions though as now we are technically having
the container run with a "services" group.

docker/Dockerfile

@@ -8,7 +8,7 @@ ARG gid=1000
 # with the same UID and GID as the current user so that the mounted directory
 # doesn't end up with files with strange permissions.
 
-RUN groupadd -g "${gid}" build && \
+RUN if [ ! $(getent group ${gid}) ]; then groupadd -g "${gid}" build; fi && \
     useradd -m -u "${uid}" -g "${gid}" build
 
-USER build:build
+USER build:${gid}