Hmm, I always have several, maybe 10 or 12 current VirtualBox machines on hand because you need to test your app with different desktops. So, my build machine is U2004 Gnome. Its a good idea to build (ie a final, release build) on a different system than you develop on, helps catch situations where your app depends on something that you just happened to have installed on your day to day machine.
And I need to test against Gnome and I'd never work, by choice, on a Gnome system.
AppImage ? I looked at that awhile ago, got some very conflicting answers about a GTK2 based AppImage, somone answered my question about where it is only to have that answer removed and replaced by a "we don't do GTK2" - no further response.
Packaging containers are generally a real pain ! My App, tomboy-ng is about a 3Meg download and uses about 6meg of diskspace. It replaces the original Tomboy that you can still install as a Flatpack, its a 700Meg download and uses 2.4G of diskspace. Anyone want to guess how much RAM it uses when running ?
Another build model that might be useful if you already make debs to distribute your app (and its open source) is Ubuntu's PPA. Despite my grizzels above about having to work with a Debian deb of FPC/Lazarus, I have built an up to date PPA based on U2004 with FPC3.2.2 and Lazarus 2.2.2, if you can make a source package, welcome to use it to build (and distribute) your app.
Davo