How to use individual configs and secrets per service

Hey there,
I would like to use docker to host multiple websites on one single server. Each can use the same nginx based image but of course, if I want to enable things like SSL they each would need slightly different configs and secrets.

I just started with docker but this one thing is still not clear to me.

How can I (programatically) scale a service but so that each new instance has it’s own set of configs and secrets?

I tried searching an answer but everybody just talks about how configs and secrets related in general for all replicas but not how I can customize this for each individual service.

Thanks!