build a multi-arch (amd64 + arm64) image

Zampler is pure Go with CGO disabled (modernc.org/sqlite), so both
architectures cross-compile from the amd64 runner at native speed with
no emulation. The runtime Dockerfile has no RUN steps, so buildx can
assemble the arm64 image on the amd64 host without QEMU too.

- Cross-compile dist/zampler-linux-{amd64,arm64} in one job.
- Dockerfile copies dist/zampler-linux-${TARGETARCH}; buildx sets
  TARGETARCH per platform leg.
- Replace `docker build`/`push` with a single `docker buildx build
  --platform linux/amd64,linux/arm64 --push`, producing a manifest list
  at :latest.
- Promote :latest to :previous with `buildx imagetools create`, which
  copies the manifest list registry-side instead of pulling a single
  arch and re-pushing.

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

.dockerignore

@@ -1,3 +1,4 @@
-# Build context only needs the prebuilt ./zampler binary and the Dockerfile.
+# Build context only needs the prebuilt per-arch binaries and the Dockerfile.
 *
-!zampler
+!dist/zampler-linux-amd64
+!dist/zampler-linux-arm64

.gitea/workflows/build.yml

@@ -12,9 +12,10 @@ jobs:
       image: golang:1.26-alpine
     env:
       CGO_ENABLED: "0"
+      IMAGE: code.aneur.in/${{ gitea.repository }}
     steps:
       - name: Install toolchain deps
-        run: apk add --no-cache nodejs npm docker-cli git tar
+        run: apk add --no-cache nodejs npm docker-cli docker-cli-buildx git tar
 
       - uses: actions/checkout@v4
 
@@ -42,8 +43,17 @@ jobs:
           npm ci
           npx vite build --outDir ../internal/server/static --emptyOutDir
 
-      - name: Build binary
-        run: go build -ldflags="-s -w" -o zampler .
+      - name: Cross-compile binaries
+        run: |
+          for arch in amd64 arm64; do
+            GOOS=linux GOARCH="$arch" \
+              go build -ldflags="-s -w" -o "dist/zampler-linux-$arch" .
+          done
+
+      - name: Set up Docker buildx
+        run: |
+          docker buildx rm multi 2>/dev/null || true
+          docker buildx create --name multi --driver docker-container --use
 
       - name: Log in to the container registry
         env:
@@ -52,14 +62,16 @@ jobs:
 
       - name: Promote current "latest" to "previous"
         run: |
-          IMAGE="code.aneur.in/${{ gitea.repository }}"
-          if docker pull "$IMAGE:latest"; then
-            docker tag "$IMAGE:latest" "$IMAGE:previous"
-            docker push "$IMAGE:previous"
+          if docker buildx imagetools inspect "$IMAGE:latest" >/dev/null 2>&1; then
+            docker buildx imagetools create -t "$IMAGE:previous" "$IMAGE:latest"
+          else
+            echo "No existing :latest to promote; skipping."
           fi
 
-      - name: Build and push "latest"
+      - name: Build and push multi-arch "latest"
         run: |
-          IMAGE="code.aneur.in/${{ gitea.repository }}:latest"
-          docker build -t "$IMAGE" .
-          docker push "$IMAGE"
+          docker buildx build \
+            --platform linux/amd64,linux/arm64 \
+            --tag "$IMAGE:latest" \
+            --push \
+            .

.gitignore

@@ -4,3 +4,4 @@
 zampler.files
 zampler.sqlite3
 internal/server/static
+dist

Dockerfile

@@ -1,12 +1,16 @@
-# The zampler binary is built in CI (.gitea/workflows/build.yml) so the compile
-# can use a persistent Go module + build cache. This image is just the runtime
-# wrapper around the prebuilt static binary.
+# The zampler binaries are cross-compiled in CI (.gitea/workflows/build.yml) so
+# the compile can use a persistent Go module + build cache and avoid emulation.
+# This image is just the runtime wrapper around the prebuilt static binary;
+# buildx sets TARGETARCH per platform and we copy the matching binary.
 #
-# For a local build: `go build -o zampler .` first, then `docker build .`.
+# For a local build:
+#   GOOS=linux GOARCH=amd64 go build -o dist/zampler-linux-amd64 .
+#   docker build --build-arg TARGETARCH=amd64 .
 
 FROM alpine:3.24
 
-COPY zampler /usr/local/bin/zampler
+ARG TARGETARCH
+COPY dist/zampler-linux-${TARGETARCH} /usr/local/bin/zampler
 
 ENTRYPOINT ["zampler"]
 CMD ["-s", "/db/zampler.sqlite3", "/data"]