Docker: trying to understand VOLUME in Dockerfile

First create a volume sample_vol

docker volume create sample_vol

My Dockerfile

FROM archlinux/base

RUN touch /root/testing  [**edited** find note at RUN below]

# VOLUME sample_vol:/root  [**edited** this will not work, because VOLUME will not accpet named volumes. So this will not mount at /root, it will mount at sample_vol:/root which does not exist]

VOLUME "/root" or VOLUME ["/root"] [**edited** this will create a local mount volume only till the time the container is running. I tried to use named volumes like VOLUME ["name:/root"] but didnt work ]

# RUN touch /root/testing   [**edited** this will not work because volume when mounted will only copy files till it got declared]

build the image

docker build -t archlinux/sample_vol .

checking whether testing file is created in sample_vol

docker run --rm -it -v=sample_vol:/tmp/myvolume archlinux/base ls /tmp/myvolume 

It does not show any file testing created

while

$ docker run --rm -it --name sample_vol archlinux/sample_vol ls /root/testing

It shows the file testing is created in the /root/ of build image

So why sample_vol is not mounted at /root and testing is created inside it.

Update: Reason i found can be due to

https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#volume

Changing the volume from within the Dockerfile: If any build steps
change the data within the volume after it has been declared, those
changes will be discarded.

Source: StackOverflow