Update build job docker image to use latest cloud-platform image

The previous cloud-platform/tools:circleci image is out of date and is using out of date ubuntu
version that is missing latest lets encrypt root certificate which was causing our slack notifications not to go out.

.circleci/config.yml

@@ -177,7 +177,7 @@ jobs:
         type: boolean
         default: false
     docker:
-      - image: ${ECR_ENDPOINT}/cloud-platform/tools:circleci
+      - image: ministryofjustice/cloud-platform-tools:2.1
     steps:
       - checkout
       - run:
@@ -218,7 +218,7 @@ jobs:
 
   cleanup_merged:
     docker:
-      - image: ${ECR_ENDPOINT}/cloud-platform/tools:circleci
+      - image: ministryofjustice/cloud-platform-tools:2.1
     steps:
       - checkout
       - run: