Ah, and with a non-federation setup you can skip all the more finicky parts of the installation (DNS, .well-known, bla bla) or just a simple server that needs to be reachable by the client.
I recommend to read a tutorial of docker and docker compose. Then the setup of stuff like synapse is a few minutes task. (Non-federation is a simple option in the config file).
Afaik, Dockge is just a front end for docker compose. So simply tagging your build (preferably with someone systematic like "local/name of software") and using that as image name should work. Or just using directly the build command of compose.
Nope