I often need to install very old ubuntu versions to replicate client's environments, or at least client's old versions of databases and such. I usually need to install these old ubuntu versions in VirtualBox. There might be a way to get them working in lxc but it's probably not worth the trouble.
ISOs for the initial install aren't hard to find, but it can then be difficult to find packages that I need. Fortunately, the Postgresql PGDG project has repositories for old versions of postgresql. But for the Ubuntu install itself, I will always need at least openssh-server (and often a bunch of other things too) and the ssh server at least isn't installed by default from desktop ISOs (I should really install server ISOs more often, but I usually forget and just grab the first amd64 ISO I see).
It turns out however that there are old Ubuntu releases at http://old-releases.ubuntu.com/ubuntu/. I'll be going there a lot. In /etc/apt/sources.list it's sufficient to replace *.ubuntu.com with old-releases.ubuntu.com and apt-get update, apt-get upgrade and apt-get install will work as normal.
