docs: add AMD Docker GPU preflight (#1168)

.env.example

@@ -145,7 +145,8 @@ SEARXNG_INSTANCE=http://localhost:8080
 #
 # AMD ROCm (requires ROCm drivers on the host and the GID of the render group):
 # COMPOSE_FILE=docker-compose.yml:docker/gpu.amd.yml
-# RENDER_GID=992
+# Find the render GID with: getent group render | cut -d: -f3
+# RENDER_GID=989
 #
 # These overlays only expose the GPU devices. The slim Odysseus image
 # still needs CUDA/ROCm userspace via Cookbook -> Dependencies (vLLM,