Validate Docker tags before build

Author: itskavin

.github/workflows/docker.yml

@@ -19,10 +19,33 @@ jobs:
         with:
           username: ${{ secrets.DOCKER_HUB_USERNAME }}
           password: ${{ secrets.DOCKER_HUB_SECRET }}
+      - name: Prepare image metadata
+        id: prep
+        run: |
+          if [ -z "$DOCKER_USER" ]; then
+            echo "::error::Secret DOCKER_HUB_USERNAME is not set." >&2
+            exit 1
+          fi
+          if [ -z "$IMAGE_REPO" ]; then
+            echo "::error::Variable REPO is not set." >&2
+            exit 1
+          fi
+          if [ -z "$IMAGE_VERSION" ]; then
+            echo "::error::Variable VERSION is not set." >&2
+            exit 1
+          fi
+
+          IMAGE_NAME="$DOCKER_USER/$IMAGE_REPO"
+          echo "image=$IMAGE_NAME" >> "$GITHUB_OUTPUT"
+          echo "version=$IMAGE_VERSION" >> "$GITHUB_OUTPUT"
+        env:
+          DOCKER_USER: ${{ secrets.DOCKER_HUB_USERNAME }}
+          IMAGE_REPO: ${{ vars.REPO }}
+          IMAGE_VERSION: ${{ vars.VERSION }}
       - name: Build and push
         uses: docker/build-push-action@v5
         with:
           context: .
           platforms: linux/amd64,linux/arm64
           push: true
-          tags: ${{ secrets.DOCKER_HUB_USERNAME }}/${{ vars.REPO }}:latest, ${{ secrets.DOCKER_HUB_USERNAME }}/${{ vars.REPO }}:${{ vars.VERSION }}-BUILD${{ github.run_number }}
+          tags: ${{ steps.prep.outputs.image }}:latest, ${{ steps.prep.outputs.image }}:${{ steps.prep.outputs.version }}-BUILD${{ github.run_number }}