The option that controls the amount of workers in uwsgi is processes not process.
Now the container spawns four processes instead of one.
Fixesmaxking/docker-mailman#368
* Add libffi to all Postorius images.
* Use Django less than a certain version.
* Fix the version constraint.
* Fix uwsgi configuration by using only processes.
* Remove the secret key.
* Use the latest tag for postorius.
* Use the latest django-mailman3 that supports Python 3.
* Use minimum version on Django-mailman3.
* Fix import errors.