Named Docker volume to share build not updating

Question

I have been asked by dev ops at the company I am working for to do something a little different with Docker then I am used too. The goal is to have 2 containers with the following responsibilities:

Container A: Node container that will build the frontend react application and place the bundle into a directory called app/dist/. When this is complete, the container will stop running.

Container B: A alpine nginx container which will server static files from /usr/share/nginx/html/app.

The files which have been built in Container A will be provided to Container B using a volume which will mount <Container A>/app/dist to <Container B>/usr/share/nginx/html/app.

Please note there is an HAProxy layer between the public accessible port and the nginx container which is container called app.

The tasks above are being orchestrated using a docker compose file which looks like the following:

version: '2'
volumes:
  webapp_build_volume: {}
services:
  webapp_build:
    build:
      context: .
      dockerfile: 'config/nginx/dockerfile-builder'
    volumes:
      - webapp_build_volume:/app/dist
      - webapp_static_volume:/app/src/app/static
  app:
    build:
      context: 'config/haproxy'
      dockerfile: 'dockerfile-app-haproxy'
    links:
      - web
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock
    ports:
      - '80:80'
      - '1936:1936'
  web:
    build:
      context: .
      dockerfile: 'config/nginx/dockerfile-web'
    environment:
      - EXCLUDE_PORTS=443
      - VIRTUAL_HOST=*
    depends_on:
      - webapp_build
    volumes:
       - webapp_build_volume:/usr/share/nginx/html/app

This is currently working only the first time the docker compose file is built. The files in the volume no longer update after the volume has been created. I have read that named volumes can not be updated after they have been established but I can not confirm this. I have found work arounds which involve running docker-compose rm --force && docker volume webapp_build rm but I would like to not have to kill the cached containers if possible since the CI service will become too slow.

Please let me know if I can clarify anything (I understand there is a lot of moving parts here). Please note I am also using the docker 2 beta though I do not see how that could change anything I have done here.

Answer 1

It's a little hard to follow, but it sounds like you are building an image, outputting files into what you believe is a volume, and trying to use that to populate a named volume used by another running container.

Most likely your confusion is that building a container doesn't mount volumes, volumes are only mounted in running containers. The named volume does have a feature where it will be populated by the contents of an image, but only when you mount a named volume that's empty. It appears you're taking advantage of this feature on the first build+run, but it won't work again on future builds. If you run your build container without a volume, you'll find that your files are there as expected.

You can easily update a named volume. Two options come to mind. One is to use your current process, but change the volume mount point to something like "/target" and as your CMD of your build container, copy the contents of your source to "/target". That would look like:

Dockerfile

...
RUN compile-cmd --output-to /local/build/dir

entrypoint.sh:

cp -a /local/build/dir/* /target/

docker-compose.yml:

version: '2'
services:
  webapp_build:
    build:
      context: .
      dockerfile: 'config/nginx/dockerfile-builder'
    volumes:
      - webapp_build_volume:/target
...

---

The second option is to not do this in the container build at all, but rather make a container with your application compiling prerequisites. Then mount your application code as a volume into this container with a CMD or ENTRYPOINT that takes the code volume contents, compiles it, and outputs it to the named volume that's also mounted. Then, instead of building the build container, you simply run the compile container with two volumes mounted.

entrypoint.sh:

compile-cmd --input-src=/source --output-to /target

docker-compose.yml:

version: '2'
services:
  webapp_build:
    volumes:
      - app/source:/source
      - webapp_build_volume:/target
...