run release build only if MY_TOKEN secret is set and not empty

.github/workflows/docker.yml

@@ -6,9 +6,22 @@ env:
   REGISTRY: ghcr.io
   IMAGE_NAME: ${{ github.repository }}
 jobs:
+  check-token:
+    runs-on: ubuntu-latest
+    outputs:
+      token: ${{ steps.token.outputs.defined }}
+    steps:
+      - id: token
+        env:
+            MY_TOKEN: ${{ secrets.MY_TOKEN }}
+        if: "${{ env.MY_TOKEN != '' }}"
+        run: echo "::set-output name=defined::true"
+        
   build:
     name: Release
     runs-on: ubuntu-latest
+    needs: [check-token]
+    if: needs.check-token.outputs.token == 'true'
     steps:
       - name: Checkout
         uses: actions/checkout@v3