Use cubicweb image from docker hub

Dockerfile

 ARG CW_INSTANCE_NAME=fresh
-FROM hub.extranet.logilab.fr/cubicweb/cubicweb:3.26-onbuild
+FROM logilab/cubicweb:3.26-onbuild
 USER root
 RUN pip install -r /src/docker/requirements.txt
 RUN cp /src/docker/pyramid.ini.j2 /pyramid.ini.j2