fix: missing env vars added

.github/workflows/docker-image.yml

@@ -6,6 +6,11 @@ on:
   pull_request:
     branches: [ "main" ]
 
+# Defines two custom environment variables for the workflow. These are used for the Container registry domain, and a name for the Docker image that this workflow builds.
+env:
+  REGISTRY: ghcr.io
+  IMAGE_NAME: ${{ github.repository }}
+
 jobs:
 
   build:
@@ -52,4 +57,3 @@ jobs:
           subject-name: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME}}
           subject-digest: ${{ steps.push.outputs.digest }}
           push-to-registry: true
-