Strange behavior with VOLUME in inherited image

I have 2 Dockerfile

s.

Here is a

:

FROM ubuntu:14.04

RUN mkdir /opt/test

RUN touch /opt/test/test.txt

VOLUME /opt/test

      

        
        
        
      

    

Here is b

:

FROM a

RUN touch /opt/test/b.txt

      

        
        
        
      

    

Then I create both of my images:

cd a && sudo docker build -t a . && cd ../b && sudo docker build -t b .

      

        
        
        
      

    

And I ran the b

image:

sudo docker run -i --rm --name b -t b /bin/bash

      

        
        
        
      

    

In my container, b

I usually don't see /opt/test/b.txt

my image b

:

root@4db094589f0d:/# ls /opt/test/
test.txt

      

        
        
        
      

    

If I remove VOLUME /opt/test

from mine a

Dockerfile

, then I will see mine correctly /opt/test/b.txt

.

Can anyone explain to me why?