build(notifier): support multi-arch image builds

Build on the native host arch (--platform=$BUILDPLATFORM) and cross-compile
the static binary per target via buildx-provided TARGETOS/TARGETARCH, so
`make docker-build` with REGISTRY set produces linux/amd64 + linux/arm64
images without emulating the Go toolchain under QEMU. Also correct the CMD
comment to the real env var (NOTIFIER_SERVER_MODE).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Dockerfile

@@ -1,5 +1,10 @@
-# Build stage
-FROM golang:1.25-alpine AS builder
+# Build stage — pinned to the build host's arch so cross-compilation
+# via GOOS/GOARCH is fast and avoids QEMU emulation of the toolchain.
+FROM --platform=$BUILDPLATFORM golang:1.25-alpine AS builder
+
+# Platform args populated by buildx for each target in a multi-platform build.
+ARG TARGETOS
+ARG TARGETARCH
 
 # Build arguments
 ARG VERSION=dev
@@ -29,9 +34,7 @@ COPY . .
 # Generate protobuf code
 RUN make proto-gen
 
-# Build binary with version information
-ARG TARGETOS=linux
-ARG TARGETARCH=amd64
+# Build binary with version information, cross-compiling to the target arch.
 RUN CGO_ENABLED=0 GOOS=${TARGETOS} GOARCH=${TARGETARCH} go build -a -installsuffix cgo \
     -ldflags "-X main.Version=${VERSION} -X main.GitCommit=${GIT_COMMIT} -X main.BuildTime=${BUILD_TIME} ${BUILD_FLAGS}" \
     -o server ./cmd/server
@@ -66,5 +69,5 @@ USER notifier
 EXPOSE 8080 50051 9090 8081
 
 # Run server (defaults to both REST and gRPC)
-# Override mode with environment variable: -e SERVER_MODE=rest or -e SERVER_MODE=grpc
+# Override mode with environment variable: -e NOTIFIER_SERVER_MODE=rest or -e NOTIFIER_SERVER_MODE=grpc
 CMD ["/app/server"]