Add GHCR credential fallback for image publish

Author: wickedyoda

.github/workflows/build.yml

@@ -219,8 +219,8 @@ jobs:
         uses: docker/login-action@v4
         with:
           registry: ghcr.io
-          username: ${{ github.actor }}
-          password: ${{ secrets.GITHUB_TOKEN }}
+          username: ${{ secrets.GHCR_USERNAME != '' && secrets.GHCR_USERNAME || github.actor }}
+          password: ${{ secrets.GHCR_TOKEN != '' && secrets.GHCR_TOKEN || secrets.GITHUB_TOKEN }}
 
       - name: Build and push main image
         uses: docker/build-push-action@v6
@@ -295,8 +295,8 @@ jobs:
         uses: docker/login-action@v4
         with:
           registry: ghcr.io
-          username: ${{ github.actor }}
-          password: ${{ secrets.GITHUB_TOKEN }}
+          username: ${{ secrets.GHCR_USERNAME != '' && secrets.GHCR_USERNAME || github.actor }}
+          password: ${{ secrets.GHCR_TOKEN != '' && secrets.GHCR_TOKEN || secrets.GITHUB_TOKEN }}
 
       - name: Build and push agent image
         uses: docker/build-push-action@v6