If RedHat doesn’t support Docker, you’re out of luck. If it’s the case that you cannot install Docker on RedHat unless you use Docker EE, then you need to use Docker EE, not use RedHat, or modify the Community version of Docker yourself so that it’ll run on RedHat. It might be easier to choose an OS that supports Docker than to re-write Docker yourself, though.
Discourse requires Docker; if you don’t have Docker, you can’t have Discourse.