Update devcontainer configuration to use pnpm and adjust volume mounting

.devcontainer/docker-compose.devcontainer.yml

@@ -1,9 +1,12 @@
-version: "3.9"
-
 services:
   workspace:
     image: mcr.microsoft.com/devcontainers/javascript-node:1-20-bookworm
     command: sleep infinity
+    # Bind mount repo root into the container. With multiple compose files, Dev
+    # Containers resolves this relative to the first file in devcontainer.json
+    # (../docker-compose.yml at repo root), so "." is the project root.
+    volumes:
+      - .:/workspace:cached
     depends_on:
       postgres:
         condition: service_healthy