Dockerfile: Pin gunicorn version
Fresh 20.0.0
release of gunicorn
broke the wsgi application factory feature (see https://github.com/benoitc/gunicorn/issues/2159).
So pin gunicorn
to version 19.9.0
until the issue gets fixed upstream.
Related to swh-environment#2080 (closed)
Migrated from D2252 (view on Phabricator)