Make docker-compose ignore folder within volume

Question

I'm using docker-compose for a Rails setup. To sync the files, I've mounted the directory like (some of the code is omitted):

app:
    build: .
    command: rails server -p 3000 -b '0.0.0.0'
    volumes:
      - .:/app

However, when running the app, I don't want the folder, tmp, to be synced. Adding the folder to dockerignore only solves the problem when building the docker image. How can I ignore the tmp folder from being synced between the host and guest when running docker compose?

Answer 1

You can add a second volume to a sub-path. It will override the contents of the existing volume. Like so :

app:
    build: .
    command: rails server -p 3000 -b '0.0.0.0'
    volumes:
      - .:/app
      - ./empty-tmp:/app/tmp

This will effectively make /app/tmp show the contents of ./empty-tmp on the host system. Instead of showing the contents of ./tmp

Answer 2

This used to be possible as described in Add a volume to Docker, but exclude a sub-folder. The solutions offered there didn't work for me anymore with docker-compose 1.25.0-rc3, file syntax v3.