contrib: Dockerfile: actually create gluon user directory

Initialize the /gluon volume with the contents of /etc/skel.

[Matthias Schiffer: extend commit message]

contrib/docker/Dockerfile

@@ -30,7 +30,7 @@ RUN mkdir /tmp/ec &&\
     mv bin/ec-linux-amd64 /usr/local/bin/editorconfig-checker &&\
     rm -rf /tmp/ec
 
-RUN useradd -d /gluon gluon
+RUN useradd -m -d /gluon gluon
 USER gluon
 
 VOLUME /gluon