Restrict merge_group trigger to checks_requested activity

Explicitly define the activity type for merge_group to prevent
unnecessary workflow runs when a merge group is destroyed.

.github/workflows/docker.yml

@@ -7,6 +7,8 @@ on:
       - 'v*.*.*'
   pull_request:
   merge_group:
+    types:
+      - checks_requested
   workflow_dispatch:
     inputs:
       use_cache:

.github/workflows/oxipng.yml

@@ -4,6 +4,8 @@ on:
   push:
   pull_request:
   merge_group:
+    types:
+      - checks_requested
   workflow_dispatch:
 
 concurrency: