Is it possible to install Ubuntu on a Mac using Bootcamp?

You have options! There are help pages for installing on Macs that show the hardware support and installation alongside OSX.

Here is the page where you select which hardware you have:

https://wiki.ubuntu.com/MactelSupportTeam/CommunityHelpPages

Just click through, select the version of Ubuntu and you'll get some installation instructions. Be sure to check the hardware compatibility too because you might not want to install if certain things aren't going to work.

The instructions for non-destructive, dual boot Mac/Ubuntu setup can be found on Ubuntu Community pages. I've have followed that instruction and installed Ubuntu 11.04 on MacBook Pro side by side with OSX. Both systems work fine.

No, Bootcamp only supports ISO files for Windows 10 or later. This was tested on MacOS Catalina 10.15 Beta.

Popup that appears if you try to attach an Ubuntu ISO file in Bootcamp

This link outlines a way to install Ubuntu as a dual-boot system on a Mac.