When creating an apt proxy, one has to specify a "distribution" name in the repository config, but on testing this field does not appear to have any effect.
1. In Nexus, configure an apt proxy repo pointing at http://archive.ubuntu.com/ubuntu. Specify distribution "focal".
2. Start an Ubuntu 20.04 image.
3. Replace /etc/apt/sources.list with a new file with this content:
4. apt-get update
5. apt-get download firefox
6. Note the version of downloaded firefox
7. Update /etc/apt/sources.list to use the bionic distro:
8. apt-get update
9. rm firefox*
10. apt-get download firefox
Notice the difference in the firefox version between point 6 and point 10.