build: publish multi-platform image (amd64, arm64, arm/v7)

The builder stage runs on the build platform and cross-compiles Rust,
with GCC cross compilers for bundled SQLite, so only the small runtime
stage runs under QEMU. The workflow sets up QEMU via the Docker Hub
mirror and pushes all three platforms under the same tags.

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

docs/DEPLOYMENT.md

@@ -14,6 +14,13 @@ are compiled at image build time.
 The numeric identity remains UID 100 / GID 101, matching the original Alpine Go
 image, so existing volume permissions are preserved across the switch.
 
+The image is multi-platform: `linux/amd64`, `linux/arm64` and `linux/arm/v7`
+(armhf) are published under the same tag, and `docker pull` selects the matching
+one. The builder stage runs on the build machine's platform and cross-compiles
+Rust (with GCC cross compilers for bundled SQLite), so only the runtime stage
+needs QEMU. Build all platforms locally with a `docker-container` buildx builder:
+`docker buildx build --platform linux/amd64,linux/arm64,linux/arm/v7 .`
+
 `docker compose up -d --build` publishes **http://localhost:8123** (host 8123,
 container 8080). `GET /health` is the liveness endpoint. Direct binary execution
 uses `:8080` by default. Place a TLS reverse proxy in front of the published port