all of these run the database in a separate container, not inside the app container. the latter would be hard to fix, but the first is just that way to make documentation easier, to be able to give you a single dockerfile. none of them use their own builds of the database server (though lemmy with its postgres variant may be a bit of an outlier).
all I do in cases like this is look up the database initialization command (in the docker compose file), run that in my primary postgres container, create a new docker network and attach it to the postgres stack and the new app’s stack (stack: the container composition defindd by the docker compose file). and then I tell the app container, usually through envvars or command line parameters embedded in the compose file, that the database server is at xy hostname, and docker’s internal DNS server will know that for xy hostname it should return the IP address of the container that is named xy, through the appropriate docker network. and also the user and pass for connection. from then, from the app’s point of view, my database server in that other container is just like a dedicated physical postgres machine you put on the network with its own cable going to a switch.
unless very special circumstances, where the app needs a custom build of postgres, they can share a single instance just fine. but in that case you would have to run 2 postgres instances even without docker, or migrate to the modified postgres, which is an option with docker too.
FlexibleToast@lemmy.world 2 weeks ago
Well, yes that’s best practice. That doesn’t mean you have to do it that way.