langchain-server: Do not expose postgresql port to host (#3431)

Apart from being unnecessary, postgresql is run on its default port,
which means that the langchain-server will fail to start if there is
already a postgresql server running on the host. This is obviously less
than ideal.

(Yeah, I don't understand why "expose" is the syntax that does not
expose the ports to the host...)

Tested by running langchain-server and trying out debugging on a host
that already has postgresql bound to the port 5432.

Co-authored-by: Sami Liedes <sami.liedes@rocket-science.ch>

langchain/docker-compose.yaml

@@ -25,5 +25,5 @@ services:
       - POSTGRES_PASSWORD=postgres
       - POSTGRES_USER=postgres
       - POSTGRES_DB=postgres
-    ports:
+    expose:
       - 5432:5432