Publish arm64 alongside amd64

muse is now the front door of a public project, and it was the only
image in that stack without an arm64 build. Raspberry Pis, arm64 NASes
and Ampere VPSes are a large share of self-hosters, so amd64-only made
the whole stack un-runnable for them — gluetun, slskd, soularr, lidarr,
navidrome and the socket proxy all already publish both.

Three parts, because changing `platforms:` alone would have failed:

- setup-qemu-action registers the binfmt handlers buildx needs to
  produce arm64 on an amd64 runner. Without it the build errors with
  "exec format error"; it does not quietly fall back to amd64.
- The web stage is pinned to --platform=$BUILDPLATFORM so the Next.js
  build runs natively once instead of being emulated per platform. It
  is the expensive step, and its output is static HTML/JS/CSS with no
  native code, so the bytes are identical either way. This is what
  makes the old comment's "3-5x slower for nothing" no longer true.
- platforms: linux/amd64,linux/arm64.

Verified locally before pushing: both platforms build in ~52s, and the
loaded arm64 image reports aarch64 with uvloop, watchfiles, httptools
and pydantic_core all importing, `import app` succeeding, and the
static UI present.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.github/workflows/release.yml

@@ -48,6 +48,14 @@ jobs:
           npm ci
           npm run build
 
+      # Registers binfmt handlers so buildx can produce a linux/arm64 image on
+      # an amd64 runner. REQUIRED by the multi-arch build below — without it
+      # buildx fails outright ("exec format error"), it does not silently fall
+      # back to amd64 only.
+      - uses: docker/setup-qemu-action@v3
+        with:
+          platforms: arm64
+
       - uses: docker/setup-buildx-action@v3
 
       - name: Log in to GHCR
@@ -73,9 +81,22 @@ jobs:
         uses: docker/build-push-action@v6
         with:
           context: .
-          # amd64 only: the only host that runs this is x86_64. Adding arm64
-          # means QEMU emulation and a 3-5x slower build for nothing.
-          platforms: linux/amd64
+          # amd64 AND arm64.
+          #
+          # This was amd64-only, and that was the right call while the only
+          # host running muse was one x86_64 box — arm64 meant paying QEMU for
+          # an image nobody would pull. That changed when muse became the
+          # front door of a public project: Raspberry Pis, arm64 NASes and
+          # Ampere VPSes are a large share of self-hosters, and every other
+          # image in that stack already publishes both. muse would have been
+          # the single thing making it un-runnable on a Pi.
+          #
+          # The old comment's cost estimate no longer applies either: the
+          # expensive stage (the Next.js build) is pinned to $BUILDPLATFORM in
+          # the Dockerfile, so it runs natively once instead of being emulated
+          # per platform. What is emulated is a pip install of prebuilt
+          # wheels.
+          platforms: linux/amd64,linux/arm64
           push: true
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}