docker - What is the purpose of VOLUME in Dockerfile

Question

I'm trying to go deeper in my understanding of Docker's volume, and I'm having an hard time to figure out the differences / use-case of:

• The docker volume create command
• The docker run -v /path:/host_path
• The VOLUME entry in the Dockerfile file

I particularly don't understand what happens if you combine the VOLUME entry with the -v flag.

Answer 1

A volume is a persistent data stored in /var/lib/docker/volumes/...

• You can either declare it in a Dockerfile, which means each time a container is started from the image, the volume is created (empty), even if you don't have any -v option.

• You can declare it on runtime docker run -v [host-dir:]container-dir.
  combining the two (VOLUME + docker run -v) means that you can mount the content of a host folder into your volume persisted by the container in /var/lib/docker/volumes/...

• docker volume create creates a volume without having to define a Dockerfile and build an image and run a container. It is used to quickly allow other containers to mount said volume.

If you had persisted some content in a volume, but since then deleted the container (which by default does not deleted its associated volume, unless you are using docker rm -v), you can re-attach said volume to a new container (declaring the same volume).

See "Docker - How to access a volume not attached to a container?".
With docker volume create, this is easy to reattached a named volume to a container.

docker volume create --name aname
docker run -v aname:/apath --name acontainer
...
# modify data in /apath
...
docker rm acontainer

# let's mount aname volume again
docker run -v aname:/apath --name acontainer
ls /apath
# you find your data back!