Build the amd64 image with QEMU 9.2.2 and only emulate the final stage

QEMU 8.x crashes running x86_64 programs on an arm64 host (QEMU issue
2168), which the 8.1.5 pin walked into, and 10.2 segfaults on the runner
as well; 9.2.2 runs node, composer and install-php-extensions on the
runner's host. The Composer and npm stages now build on the build
machine's platform: their output is the same for every target, so a
multi-arch build runs them once and natively.

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

.gitea/workflows/docker.yml

@@ -67,12 +67,14 @@ 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.
+      # The runner is an arm64 server, so the amd64 image's final stage is emulated. QEMU 8.x
+      # crashes running x86_64 programs on an arm64 host (QEMU issue 2168, "QEMU internal
+      # SIGSEGV {code=MAPERR, addr=0x20}") and 10.2 segfaults on this runner too; 9.2.2 was
+      # checked on the runner's host: node, composer and install-php-extensions all run.
       - name: Set up QEMU
         uses: docker/setup-qemu-action@v3
         with:
-          image: tonistiigi/binfmt:qemu-v8.1.5
+          image: tonistiigi/binfmt:qemu-v9.2.2
 
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3