Make sure next builds run as root

This keeps the chain as it was (now that a devel user is added to the
base image).

Dockerfile

@@ -5,3 +5,7 @@ WORKDIR /
 RUN wget "https://git.simply-life.net/packages/ubuntu/trusty/qommando-master.tar.bz2" -O /tmp/qommando-master.tar.bz2 \
     && tar -jxvf /tmp/qommando-master.tar.bz2 \
     && rm /tmp/qommando-master.tar.bz2
+
+# Don't use devel user on the next image builds
+ONBUILD USER root
+ONBUILD WORKDIR /