ci: remove pull_request trigger from docker-build workflow (#11)

Remove pull_request trigger to prevent unnecessary builds on PRs.
The workflow will now only run on main branch pushes and releases.

Author: hrntknr

.github/workflows/docker-build.yaml

@@ -4,9 +4,6 @@ on:
   push:
     branches:
       - main
-  pull_request:
-    branches:
-      - main
   release:
     types: [published]