Isolate node_modules in Dev Container with a named volume

Bind-mounting the whole repo meant a host-installed node_modules
(e.g. from macOS/Windows) shadowed the container's own install,
causing native modules like Prisma's query engine to be built for
the wrong platform.

.devcontainer/docker-compose.yml

@@ -5,9 +5,13 @@ services:
       dockerfile: .devcontainer/Dockerfile
     volumes:
       - ..:/workspaces/ghostfolio:cached
+      - node_modules:/workspaces/ghostfolio/node_modules
     command: sleep infinity
     depends_on:
       postgres:
         condition: service_healthy
       redis:
         condition: service_healthy
+
+volumes:
+  node_modules: