fix(docker): nginx fails to start on hosts without IPv6 (#2027)

* fix(docker): nginx fails to start on hosts without IPv6

- Detect IPv6 support at runtime and remove `listen [::]` directive
  when unavailable, preventing nginx startup failure on non-IPv6 hosts
- Use `exec` to replace shell with nginx as PID 1 for proper signal
  handling (graceful shutdown on SIGTERM)
- Reformat command from YAML folded scalar to block scalar (no
  functional change)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix(docker): harden nginx startup script (Copilot review feedback)

Add `set -e` so envsubst failures exit immediately instead of starting
nginx with an incomplete config. Narrow the sed pattern to match only
the `listen [::]:2026;` directive to avoid accidentally removing future
lines containing [::].

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

docker/docker-compose-dev.yaml

@@ -69,10 +69,15 @@ services:
     environment:
       - LANGGRAPH_UPSTREAM=${LANGGRAPH_UPSTREAM:-langgraph:2024}
       - LANGGRAPH_REWRITE=${LANGGRAPH_REWRITE:-/}
-    command: >
-      sh -c "envsubst '$$LANGGRAPH_UPSTREAM $$LANGGRAPH_REWRITE'
-      < /etc/nginx/nginx.conf.template > /etc/nginx/nginx.conf
-      && nginx -g 'daemon off;'"
+    command:
+      - sh
+      - -c
+      - |
+        set -e
+        envsubst '$$LANGGRAPH_UPSTREAM $$LANGGRAPH_REWRITE' \
+          < /etc/nginx/nginx.conf.template > /etc/nginx/nginx.conf
+        test -e /proc/net/if_inet6 || sed -i '/^[[:space:]]*listen[[:space:]]\+\[::\]:2026;/d' /etc/nginx/nginx.conf
+        exec nginx -g 'daemon off;'
     depends_on:
       - frontend
       - gateway