Bump django from 3.1.7 to 3.1.8 in /containers/backend

Bumps [django](https://github.com/django/django) from 3.1.7 to 3.1.8.
- [Release notes](https://github.com/django/django/releases)
- [Commits](https://github.com/django/django/compare/3.1.7...3.1.8)

Signed-off-by: dependabot[bot] <support@github.com>

containers/backend/requirements.txt

@@ -1,4 +1,4 @@
-Django==3.1.7
+Django==3.1.8
 django-cors-middleware==1.5.0
 djangorestframework==3.12.2
 djangorestframework-simplejwt==4.6.0