Normalize image naming convention in GitHub Actions workflow

Author: Spomky

.github/workflows/build.yml

@@ -7,7 +7,6 @@ on:
   workflow_dispatch:
 
 env:
-  IMAGE_NAME: ghcr.io/${{ github.repository }}
   RUNNER_VERSION: 2.327.1
 
 jobs:
@@ -16,6 +15,9 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
+      - name: Normalize image name
+        run: echo "IMAGE_NAME=$(echo ghcr.io/${{ github.repository_owner }}/gh-runner | tr '[:upper:]' '[:lower:]')" >> $GITHUB_ENV
+
       - name: Log in to GHCR
         run: echo "${{ secrets.GITHUB_TOKEN }}" | docker login ghcr.io -u "${{ github.actor }}" --password-stdin