Here's the Reason why your dockerfile is structured that way.
You have to create a dockerfile and the base image should be devopsedu/webapp. And you have to write dockerfile in such a way that it should run the PHP application.
The below sample docker file which uses devopssedu/webapp as the base image and then it runs the PHP application.
ADD proj /var/www/html
RUN rm /var/www/html/index.html
CMD apachectl -D FOREGROUND
/var/www/html is the directory where your website files are stored. These files include the codes, images etc used for the website.
The application code is moved or copied inside the container and a docker image is build. An image is created.
Now when you run this image, the php application will get executed.