Run the docker workflow by schedule on master and manually for PRs

.github/workflows/docker.yml

@@ -1,13 +1,13 @@
 name: Docker image
 
 on:
-  pull_request: {}
-  push:
-    branches:
-      - master
   release:
     types:
       - published
+  schedule:
+    # for master, every day at midnight
+    - cron: "0 0 * * *"
+  workflow_dispatch:
 
 jobs:
   docker: