git-deamon: Make sure to create the git-daemon user

Recent debian packages moved creation of the git-daemon user from
the main "git" package into the "git-daemon-run" package.  We want
the user without the latter...

Change-Id: I42f68ba31d32574a01d1372d8cc5239c420e816f

git-daemon/Dockerfile

@@ -8,6 +8,8 @@ RUN apt-get update -qq
 
 RUN apt-get install -qqy git
 
+RUN useradd -u 30001 -g ssh git-daemon
+
 ADD git-daemon.sh /usr/bin/git-daemon.sh
 VOLUME /git