How to change a file in /etc inside a docker container using dockerfile?

Question

I have a docker application, I want to append the word "Hellow" to the passwd file inside the docker container. I made this change to Dockerfile:

RUN echo "Hellow" >> /etc/passwd

But the word Hellow is not showing after I run cat /etc/passwd by using docker exec -u 0 -it my_php-apache_1 bash and then cat /etc/passwd command.