fix(DockerFile): update pip during installation

Fix #10 (closed)

Before this MR, the following failed:

docker run --rm -it --entrypoint='/bin/bash'  logilab/cubicweb:dev
pip install psycopg2-binary
pip --version # return 18.1

After this MR, the same works:

docker run --rm -it --entrypoint='/bin/bash'  logilab/cubicweb:dev
pip install psycopg2-binary
pip --version # returns 21.1.2

To test it, build an image locally:

In build.py :

-     images = get_cubicweb_images(["stretch", "buster"], ["py27"])                                                                                                                                                 
-     images += get_cubicweb_images(["stretch"], ["py35"])                                                                                                                                                          
-     images += get_cubicweb_images(["buster"], ["py37"])                                                                                                                                                           
+     images = []                                                                                                                                                                                                   

This will build only the dev version.

then build it with python3 build.py