I did not find an explanation on how to use one Redis for multiple instances in that how-to, but this seems to work out of the box when multisite configuration is available?
Do I understand correctly ?
a) Multisite runs the same docker-image for multiple forums, using one redis and one postgres server.
b) Using one redis for multiple discourse instances (using the same image or not) is not possible, since without turning multisite on, no prefix will be added to the redis keys.
c) There is no other way to enforce a redis key prefix.
d) Enforcing different redis key prefixes for different discourse images would allow using one redis server.
e) General support for enforcing a general redis prefix would need several core changes.