Emulate the amd64 image with QEMU 8 on the arm64 runner

Recent QEMU segfaults compiling PHP extensions for amd64 on the runner's
6.8 kernel (docker/buildx#3170); QEMU 8.1.5 is the version reported to
work.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01V9NnLxnPp8vaaurb3Z1MFy

.gitea/workflows/docker.yml

@@ -67,8 +67,12 @@ jobs:
       - name: Checkout code
         uses: actions/checkout@v6
 
+      # The runner is an arm64 server, so the amd64 image is emulated. On its 6.8 kernel, recent
+      # QEMU builds segfault compiling PHP extensions (docker/buildx#3170); QEMU 8 is pinned.
       - name: Set up QEMU
         uses: docker/setup-qemu-action@v3
+        with:
+          image: tonistiigi/binfmt:qemu-v8.1.5
 
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3