fix(docker): cross-compile the image instead of running Go under QEMU

Building the image for linux/amd64 on an arm64 host (Apple Silicon)
kept failing at 'go mod download' with 'unexpected EOF'.

Two changes:

1. The builder stages had no --platform, so the Node and Go toolchains
   ran under QEMU emulation of the target arch. Pin frontend-builder
   and backend-builder to $BUILDPLATFORM and cross-compile: add
   TARGETOS/TARGETARCH args and set them on 'go build'. The binary is
   CGO_ENABLED=0, so this is a clean pure-Go cross-compile - much
   faster, and the emulated networking that dropped module fetches
   with EOF is gone. The frontend output is JS (arch-neutral), so it
   is safe to build on the host arch too.

2. Add go module and build cache mounts, so a retry after a network
   blip goes on from where it stopped instead of starting over.

The runtime image and app behavior are unchanged.

Dockerfile

@@ -18,7 +18,9 @@ ARG NPM_CONFIG_REGISTRY=
 # -----------------------------------------------------------------------------
 # Stage 1: Frontend Builder
 # -----------------------------------------------------------------------------
-FROM ${NODE_IMAGE} AS frontend-builder
+# --platform=$BUILDPLATFORM: the frontend output is JS (arch-neutral), so build
+# it on the native host arch instead of under QEMU emulation for the target.
+FROM --platform=${BUILDPLATFORM} ${NODE_IMAGE} AS frontend-builder
 ARG NPM_CONFIG_REGISTRY
 
 WORKDIR /app/frontend
@@ -44,7 +46,11 @@ RUN pnpm run build
 # -----------------------------------------------------------------------------
 # Stage 2: Backend Builder
 # -----------------------------------------------------------------------------
-FROM ${GOLANG_IMAGE} AS backend-builder
+# --platform=$BUILDPLATFORM: run the Go toolchain on the native host arch and
+# cross-compile to the target arch below. The binary is CGO_ENABLED=0, so this
+# is a clean pure-Go cross-compile — no QEMU emulation of go mod download / go
+# build (emulated networking here was dropping module fetches with EOF).
+FROM --platform=${BUILDPLATFORM} ${GOLANG_IMAGE} AS backend-builder
 
 # Build arguments for version info (set by CI)
 ARG VERSION=
@@ -52,6 +58,9 @@ ARG COMMIT=docker
 ARG DATE
 ARG GOPROXY
 ARG GOSUMDB
+# Populated by buildx from the --platform target (e.g. linux/amd64).
+ARG TARGETOS
+ARG TARGETARCH
 
 ENV GOPROXY=${GOPROXY}
 ENV GOSUMDB=${GOSUMDB}
@@ -63,7 +72,10 @@ WORKDIR /app/backend
 
 # Copy go mod files first (better caching)
 COPY backend/go.mod backend/go.sum ./
-RUN go mod download
+# Cache mount keeps the module cache across builds so a transient CDN blip on
+# retry resumes instead of re-fetching every zip from scratch.
+RUN --mount=type=cache,id=sub2api-gomod,target=/go/pkg/mod \
+    go mod download
 
 # Copy backend source first
 COPY backend/ ./
@@ -73,10 +85,12 @@ COPY --from=frontend-builder /app/backend/internal/web/dist ./internal/web/dist
 
 # Build the binary (BuildType=release for CI builds, embed frontend)
 # Version precedence: build arg VERSION > exact git tag > cmd/server/VERSION
-RUN VERSION_VALUE="${VERSION}" && \
+RUN --mount=type=cache,id=sub2api-gomod,target=/go/pkg/mod \
+    --mount=type=cache,id=sub2api-gobuild,target=/root/.cache/go-build \
+    VERSION_VALUE="${VERSION}" && \
     if [ -z "${VERSION_VALUE}" ]; then VERSION_VALUE="$(./scripts/resolve-version.sh)"; fi && \
     DATE_VALUE="${DATE:-$(date -u +%Y-%m-%dT%H:%M:%SZ)}" && \
-    CGO_ENABLED=0 GOOS=linux go build \
+    CGO_ENABLED=0 GOOS=${TARGETOS:-linux} GOARCH=${TARGETARCH} go build \
     -tags embed \
     -ldflags="-s -w -X main.Version=${VERSION_VALUE} -X main.Commit=${COMMIT} -X main.Date=${DATE_VALUE} -X main.BuildType=release" \
     -trimpath \