What is the purpose of VOLUME in Dockerfile

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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!

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