Add multi-arch Docker build and docker-compose stack

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.

.gitignore

@@ -1,5 +1,11 @@
 build/
 /*.yaml
+# docker-compose.yaml is an exception to the /*.yaml rule above; it's
+# tracked as part of the container deployment.
+!/docker-compose.yaml
+# .env holds (potentially secret) docker-compose runtime settings and
+# must never be committed. .env.example is the tracked template.
+.env
 docs/implementation/
 tests/out/
 tests/.venv/