Introduce a multi-stage Alpine Dockerfile that cross-compiles via
buildx ($BUILDPLATFORM -> $TARGETARCH) so a single invocation produces
both linux/amd64 and linux/arm64 images without a qemu-emulated
builder. `make docker` loads the native-arch image locally for smoke
tests; `make docker-push` publishes a multi-arch manifest. Ship a
docker-compose.yaml with opt-in profiles for maglevd/frontend and a
.env.example template so operators can mirror /etc/default/vpp-maglev
muscle memory into containers.
