Dockerfile: Pin gunicorn version

Latest gunicorn release (20.0.0) broke the wsgi application factory feature
(see benoitc/gunicorn#2159).

So pin gunicorn version to 19.9.0 until the issue gets fixed upstream.

Closes T2080

Dockerfile

@@ -22,7 +22,7 @@ USER swh
 RUN python3 -m venv /srv/softwareheritage/venv
 ENV PATH="/srv/softwareheritage/venv/bin:${PATH}"
 RUN pip install --upgrade pip setuptools wheel
-RUN pip install gunicorn
+RUN pip install gunicorn==19.9.0
 
 RUN pip install \
         swh-core[db,http] \