Make PORT a Dockerfile argument

Is your feature request related to a problem? Please describe.

I would like to use this for CI testing and container-based development of a project, but that project already uses port 3000 in dev mode, which conflicts with this.

Describe the solution you'd like

I'd like the PORT in the Dockerfile to be changed to a build parameter, and then used as a variable to configure the port rather than having the 3000 be hard-coded and/or assumed in the configuration and program setup.

Describe alternatives you've considered

I tried to use port remapping, but the problem is that the browser in the container still need to be able to connect to port 3000, so it doesn't matter if the port isn't 3000 outside the container. Our project knows what port it's served on and uses that to construct urls, so it doesn't really work if its port has been remapped.

Another possibility is just changing the default to 3001, to make it less likely to conflict with other projects. Making it a parameter seems like a better solution though.

Additional context

I guess it's significant that the default dev port for Rails projects is 3000, so this will conflict with all of those while it's also 3000. The same may go for other platforms.