use volume defined in Dockerfile from docker-compose

Question

I have for example this service and volume defined in my docker-compose file

  postgres:
    image: postgres:9.4
    volumes:
      - db_data:/var/lib/postgresql/data
  volumes:
    blue_prod_db:
      driver: rancher-nfs

Then. if you define a volume inside a Dockerfile like this:

RUN mkdir /stuff
COPY ./stuff/* /stuff/
VOLUME /stuff

How can you later access it through the docker-compose configuration and add it to a container?

Answer 1

When configured in the Dockerfile, a volume will result in any container started from that image, including temporary containers later in the build process from the RUN command, to have a volume defined at the specified location, e.g. /stuff. If you do not define a source for that volume at run time, you will get an anonymous volume created by docker for you at that location. However, you can always define a volume with a source at run time (even without the volume being defined) by specifying the location in your compose file:

version: "3"
services:
  app:
    image: your_image
    volumes:
      - data:/stuff
volumes:
  data:

Note that there are two volumes sections, one for a specific service that specifies where the volume is mounted inside the container, and another at the top level where you can specify the source of the volume. Without specifying a source, you'll get a local volume driver with a directory under /var/lib/docker bind mounted into the container.

I do not recommend specifying volumes inside the Dockerfile in general, it breaks the ability to extend the image in later steps for child images, and clutters the filesystem with anonymous volumes that are not easy to track back to their origin. It's best to define them at runtime with something like a compose file.