All of our builds start as ISOs. We then pull the ISO apart to create the LXC templates (and the other builds such as OVA, etc - you can see all the builds here). Whilst that may seem a little bizarre perhaps at first glance , we do it that way to ensure that the different builds are as close as practical to one another.

If you wish to do things the way we do, then build your desired appliance as an ISO on TKLDev via buildtasks - explicitly using bt-iso (FWIW I published a blog post recently about building Debian Buster based apps on v15.x TKLDev). Once you have your ISO built (and it will generate a few other files too), then you can turn that ISO into a LXC template via bt-container (probably poorly named...). For example to build a LXC template of v16.0rc1 Core (after following the blog post):

./bt-container core-16.0rc1-buster-amd64

Please note that I'm yet to look at template building for v16.x, so I can't guarantee that it will work 100% (although I suspect that it will mostly, if not completely).

Alternatively, seeing as all our appliances are based on Debian, you could add our repos and install the desired packages from our repo. Please note that for Buster, I haven't yet pushed the packages to the 'buster main' repo, so you'll need to enable our 'buster-testing' repo. I hope to build the main packages to 'buster main' early next week (when I build v16.0rc1 it won't have testing repo enabled by default).