docker: only build branches main, dev

Otherwise branches will accumulate in 'Recent tagged image version'
https://github.com/vogler/free-games-claimer/pkgs/container/free-games-claimer

Author: vogler

.github/workflows/docker.yml

@@ -1,9 +1,9 @@
 name: Build and push Docker image (amd64, arm64 to hub.docker.com and ghcr.io)
 
 on:
-  workflow_dispatch: # allow manual trigger
-  push: # build for each branch
-    # branches: ["main"]
+  workflow_dispatch: # allows manual trigger
+  push: # push on branch
+    branches: [main, dev]
     paths: # ignore changes to .md files
       - '**'
       - '!*.md'