stacks/fish-s2: build with target=server (multi-stage Dockerfile, default builds webui-only)

Fourth fish-s2 attempt got past build + checkpoints, then container
crashlooped silently again. Diagnosis: the upstream docker/Dockerfile
is multi-stage with `webui` and `server` targets; without specifying
a target, docker builds the LAST stage (webui — gradio-only, no
start_server.sh, no API server). start_server.sh is the entrypoint
script that lives only in the `server` stage.

Confirmed by `cat /app/start_server.sh` inside the built image:
"No such file or directory."

Upstream's compose.yml uses target: server on its server service —
doing the same here.

stacks/fish-s2/compose.yaml

@@ -34,6 +34,11 @@ services:
       # 403s on anonymous pulls. Build from source via docker/Dockerfile
       # instead.
       dockerfile: docker/Dockerfile
+      # Multi-stage Dockerfile — `server` stage installs start_server.sh
+      # and exposes the API. The default last stage is `webui` (gradio
+      # only), which crashloops silently because start_server.sh isn't
+      # there. Target=server is what upstream's compose.yml does.
+      target: server
       args:
         # Build args mirror upstream compose.base.yml defaults.
         # CUDA_VER 12.9 + UV_EXTRA cu129 = the CUDA 12.9 PyTorch wheels.