What does docker run -p 8080:3000 myapp do?

"Forwards host port 8080 to container port 3000" is right. The format is HOST:CONTAINER. So requests to port 8080 on the host go to port 3000 in the container.

"Forwards container port 8080 to host port 3000" mixes up the order, which is an easy mistake. The app inside still listens only on 3000. It knows nothing about 8080.