SKALE-4128 run release build only on merge to stable

.github/workflows/dockerimagerelease.yml

 name: Build, test and push release SGX container
 on:
   workflow_dispatch:
-  push:
+  pull_request:
+    types: [closed]
+    branches:
+      - stable
 jobs:
   build:
     runs-on: ubuntu-18.04