Update Celery Docker image to Python 3.9

https://docs.celeryproject.org/en/stable/whatsnew-5.1.html

While the Celery Docker image has been upgraded to Python 3.9, the
application is still running Celery 4. Further updates to the source
code will be needed before upgrading from Celery 4.

{{cookiecutter.project_slug}}/backend/celeryworker.dockerfile

@@ -1,4 +1,4 @@
-FROM python:3.7
+FROM python:3.9
 
 WORKDIR /app/