feat: initial dflash-server docker packaging

Multi-stage CUDA build of the native dflash_server from
Luce-Org/lucebox-hub (pinned at 42f36f1). Models are not baked
into the image; mount /models at runtime.

- Dockerfile: nvidia/cuda:12.6.0 devel -> runtime, CUDA_ARCH build-arg
  (default sm_86), libcuda.so.1 stub symlink + -rpath-link fix
- docker-compose.yml: reference service with ./models:/models:ro
- Makefile: submodules / doctor / build / run / shell / up-down-logs /
  push / clean. push targets gitea.va.reichard.io/evan
- README + .dockerignore + .gitignore

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "lucebox-hub"]
+	path = lucebox-hub
+	url = https://github.com/Luce-Org/lucebox-hub.git