Able to override the root permission of a readonly file in `Docker` with a non-sudo user

I was performing some experiments in Docker and found a strange behaviour.

I was able to override the ownership of a file created with the root user inside the Docker with another user without root permissions.

Below are the steps to reproduce it:

$> docker run -dit ubuntu:16.04 bash $> docker exec -it cont_id bash $> apt update && apt install -y vim $> useradd cp -m $> vim /home/cp/hello.txt # Write some text and save it $> su cp $> cd ~/ && ls -latr; # Will list hello.txt with user and group as root $> vim hello.txt # Write some text and try saving it normally which will fail. # Try saving it with `:wq!` 

Voila, it is saved and the user and group to which the file belongs also change to the new user.

I have done a terminal recording for this and the same is posted here.