ci: Build the Docker image's static site natively instead of under QEMU

The multi-platform Docker build ran pnpm install and pnpm build for
linux/arm64 under QEMU emulation. QEMU intermittently crashes there
("qemu: uncaught target signal 4 (Illegal instruction)") or stalls, and
buildkit then waits until GitHub cancels the job at 6 hours: 16
cancelled runs in September, including a develop push.

The build output is a static site, identical on every architecture, so
the node stages now run on $BUILDPLATFORM and only the nginx stage
(COPY only) is built per architecture. Also caps the job at 30 minutes
so any future hang fails fast.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.github/workflows/docker-publish.yml

@@ -17,6 +17,8 @@ on:
 jobs:
   docker:
     runs-on: ubuntu-latest
+    # A healthy build takes a few minutes; fail fast instead of hanging for 6 hours.
+    timeout-minutes: 30
     permissions:
       contents: read
       packages: write

Dockerfile

@@ -1,4 +1,7 @@
-FROM docker.io/library/node:24.16.0-alpine3.22 AS mermaid-live-editor-dependencies
+# The build output is a static site, so build it once on the runner's native
+# platform instead of under QEMU emulation; only the nginx stage below is
+# per-architecture.
+FROM --platform=$BUILDPLATFORM docker.io/library/node:24.16.0-alpine3.22 AS mermaid-live-editor-dependencies
 
 RUN apk --no-cache add build-base git python3 && \
     rm -rf /var/cache/apk/*