I resolved this situation (where a docker container is serving Una), and am posting it here in case it helps someone else.

Edit /etc/apache2/ports.conf, add Listen 8000; and edit mysite.conf to: <VirtualHost *:80 *:8000>

With these changes, within the docker container, localhost:8000 works as well.

If I were to do this from scratch, I would configure apache2 to server on 8000 only, and docker map 8000->8000, so from within the container and from outside the container, localhost:8000 would be the correct address. I would not need port 80.