[docker] fix /etc/cubicweb.d not writable by cubicweb user

Dockerfile

@@ -5,6 +5,7 @@ RUN pip install -e /src
 RUN pip install -r /src/docker/requirements.txt
 RUN cp /src/docker/pyramid.ini.j2 /pyramid.ini.j2
 RUN install -d -o cubicweb -g cubicweb /etc/cubicweb.d/fresh/bfss
+RUN chown -R cubicweb: /etc/cubicweb.d
 USER cubicweb
 ENV WITH_TRUSTEDAUTH=false
 ENV CW_INSTANCE=fresh