Default to port 3000 while allowing override

[elia]

With this configuration bin/dev will use port 3000, but bin/dev -p 3001 will correctly start the server on port 3001.

[dhh]

This would overwrite the port ENV set by a Dockerfile, so don't think that works.

[elia]

@dhh updated so it respects a PORT environment variable set from outside

[dhh]

could you do the same on cssbundling-rails? thanks

[elia]

@dhh sure! Here it is https://github.com/rails/cssbundling-rails/pull/124