A Note on EXPOSE

This was a point of confusion for me so I'm going to try to clear it up for you. There is an instruction called EXPOSE <port number> that its intended use is to expose ports from within the container to the host machine. However if we don't do the -p 3000:3000 it still isn't exposed so in reality this instruction doesn't do much. You don't need EXPOSE.

There are two caveats to that. The first is that it could be useful documentation to say that "I know this Node.js service listens on port 3000 and now anyone who reads this Docekrfile will know that too." I would challenge this that I don't think the Dockerfile is the best place for that documentation

The second caveat is that instead of -p 3000:3000 you can do -P. This will take all of the ports you exposed using EXPOSE and will map them to random ports on the host. You can see what ports it chose by using docker ps. It'll say something like 0.0.0.0:32769->3000/tcp so you can see in this case it chose 32769. Again, I'd prefer to be deliberate about which ports are being mapped.