fix: declare image name in one place

.github/workflows/build.yml

@@ -17,7 +17,7 @@ on:
   workflow_dispatch:
 
 env:
-  IMAGE_BASE_NAME: startingpoint
+  IMAGE_NAME: startingpoint
   IMAGE_REGISTRY: ghcr.io/${{ github.repository_owner }}
 
 jobs:
@@ -31,7 +31,6 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        image_name: [startingpoint]
         major_version: [37]
         include:
           - major_version: 37
@@ -44,7 +43,7 @@ jobs:
 
       - name: Matrix Variables
         run: |
-          echo "IMAGE_NAME=${{ format('{1}', matrix.image_name, env.IMAGE_BASE_NAME) }}" >> $GITHUB_ENV
+          echo "IMAGE_NAME=${{ env.IMAGE_NAME }}" >> $GITHUB_ENV
 
       - name: Generate tags
         id: generate-tags