[docker] don't use deprecated onbuild images

Dockerfile

-ARG CW_INSTANCE_NAME=fresh
-FROM logilab/cubicweb:dev-onbuild
+FROM logilab/cubicweb:dev
 USER root
+COPY . /src/
+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
 USER cubicweb
 ENV WITH_TRUSTEDAUTH=false
+ENV CW_INSTANCE=fresh
+RUN docker-cubicweb-helper create-instance