Removed stale container image cache location.

Hopefully we'll reinstate this in the future, but make use of GitHub
Container Registry. It'll be easier to maintain and we should be able to
publish all versions on a regular basis from a GitHub Actions workflow.

Author: ngnpope

compose.yml

@@ -1,15 +1,13 @@
 ---
 
 x-base: &base
-  image: djangobox/django-docker-box:${PYTHON_IMPLEMENTATION}-${PYTHON_VERSION}
+  image: django-docker-box:${PYTHON_IMPLEMENTATION}-${PYTHON_VERSION}
   build:
     context: .
     dockerfile: ${PWD}/Containerfile
     args:
       - PYTHON_IMPLEMENTATION=${PYTHON_IMPLEMENTATION}
       - PYTHON_VERSION=${PYTHON_VERSION}
-    cache_from:
-      - djangobox/django-docker-box:${PYTHON_IMPLEMENTATION}-${PYTHON_VERSION}
     additional_contexts:
       src: ${DJANGO_PATH:-../django}
   volumes: