ci: run amd64 docker build on PRs

Author: sveitser

.github/workflows/espresso-docker.yml

@@ -16,6 +16,7 @@ run-name: Docker build CI triggered from @${{ github.actor }} of ${{ github.head
 on:
   workflow_dispatch:
   merge_group:
+  pull_request:
   push:
     branches:
       - master
@@ -32,6 +33,8 @@ concurrency:
 
 jobs:
   docker_build:
+    # Don't run the arm build that uses a non-free runner on PRs
+    if: ${{ !(matrix.platform == 'linux/arm64' && github.event_name == 'pull_request') }}
     strategy:
       matrix:
         platform: [linux/amd64, linux/arm64]