Dockerfile - update pip

Dockerfile

@@ -8,5 +8,6 @@ RUN set -ex \
     && apt-get install -y curl build-essential python3-dev gettext make \
     && rm -rf /var/lib/apt/lists/* \
     && echo "[global]\ntrusted-host=repodipory.u-strasbg.fr" > /etc/pip.conf \
+    && pip install --upgrade pip \
     && pip install tox sphinx