ATM, port binding is 1 to 1, where both sides are ints. Which means, you can easilly map port 8080 to 9000.

But docker also supports specifying port ranges, that means that:

• You can map a range of ports from the container to the host, for instance 8000-9000:8000-9000
• You can map a particular port from the container to a range of ports, for instance 8000-9000:8080

In order to do that, the port bindings need to support also strings and not only integers.