Update IMAGE_NAME in CI/CD workflows to use a fixed repository name

Author: bechoforest

.github/workflows/ci.yml

@@ -8,7 +8,7 @@ on:
 
 env:
   REGISTRY: ghcr.io
-  IMAGE_NAME: ${{ github.repository }}
+  IMAGE_NAME: bxdavies/mustmail
 
 jobs:
   version:

.github/workflows/dev.yml

@@ -9,7 +9,7 @@ on:
 
 env:
   REGISTRY: ghcr.io
-  IMAGE_NAME: ${{ github.repository }}
+  IMAGE_NAME: bxdavies/mustmail
 
 jobs:
   build-and-push-image: