Clear out week old images from cache

.gitlab-ci.yml

@@ -43,3 +43,12 @@ build-image:
   after_script:
     - docker system df
     - docker system prune -f
+    - |
+      docker images --format '{{.Repository}}:{{.Tag}} {{.ID}} {{.CreatedSince}}' | grep 'weeks' | while read -r line; do
+          image_id=$(echo $line | awk '{print $2}')
+          if ! docker ps -a --format '{{.Image}}' | grep -q $(echo $line | awk '{print $1}'); then
+              docker rmi $image_id
+          fi
+      done
+
+