I'm basically using Docker on Mac as an alternative to VMWare Fusion with a much faster startup startup time and more flexible directory sharing.

I want to avoid port mapping because I already have things on the Mac using the ports that my things in the container are using.

I have a test environment that can run in a VM, container, or an actual machine like an RPi. It has copies of most of our live systems, with customer data removed. It is designed so that as much as possible things inside it run with the exact same configuration they do live. The web sites in then are on ports 80 and 443, MySQL/MariaDB is on 3306, and so on. Similarly, when I'm working on something that needs to access those services from outside the test system I want to as much as possible use the same configuration they will use when live, so they want to connect to those same port numbers.

Thus I need the test environment to have its own IP that the Mac can reach.

Or maybe not...I just remembered something from long ago. I wanted a simpler way to access things inside the firewall at work than using whatever crappy VPN we had, so I made a poor man's VPN with ssh. If I needed to access things on say port 80 and 3306 on host foo at work, I'd ssh to somewhere I could ssh to inside the firewall at work, setting that up to forward say local 10080 and 13306 to foo:80 and foo:3306. I'd add an /etc/hosts entry at foo giving it some unused address like 10.10.10.1. Then I'd use ipfw to set it up so that any attempt to connect to 10.10.10.1:80 or 10.10.10.1:3306 would get forwarded to 127.0.0.1:10080 or 127.0.0.1:13306, respectively. That worked great until Apple replaced ipfw with something else. By then we had a decent VPN for work and so I no longer need my poor man's VPN and didn't look into how to do this in whatever replaced ipfw.

Learning how to do that in whatever Apple now uses might be a nice approach. I'll have to look into that.