Node.js + Docker Compose: node_modules disappears

I'm attempting to use Docker Compose to bring together a number of Node.js apps in my development environment. I'm running into an issue, however, with node_modules.

Here's what's happening:

1. npm install is run as part of the Dockerfile.
2. I do not have node_modules in my local directory. (I shouldn't because the installation of dependencies should happen in the container, right? It seems to defeat the purpose otherwise, since I'd need to have Node.js installed locally.)
3. In docker-compose.yml, I'm setting up a volume with the source code.
4. docker-compose build runs fine.
5. When I docker-compose up, the node_modules directory disappears in the container — I'm assuming because the volume is mounted and I don't have it in my local directory.

How do I ensure that node_modules sticks around?

Dockerfile

FROM       node:0.10.37

COPY       package.json /src/package.json

WORKDIR    /src

RUN        npm install -g grunt-cli && npm install

COPY       . /src

EXPOSE     9001

CMD        ["npm", "start"]

docker-compose.yml

api:
  build: .
  command: grunt
  links:
    - elasticsearch
  ports:
    - "9002:9002"
  volumes:
    - .:/src

elasticsearch:
  image: elasticsearch:1.5