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

What I ended up doing was to simply map all the folders which I wanted to sync:

- ./app:/app/app
- ./lib:/app/lib
- ./log:/app/log
- ./spec:/app/spec

Which seems to solve the problem

Answer 2

You can use a data volume to store tmp, as data volumes copy in the data from the built docker image before the app directory is mounted

volumes:
- .:/app
- /path/to/tmp

Answer 3

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