OK for me to get it work in Plesk Onyx I had to do a few things differently to get it to work correctly.
Firstly get the image of Rocker chat that you have installed removed so you have nothing installed in the Docker panel.
Make sure you have your domains and sub domains added to the Plesk panel you should be able to auto secure with Letsencrypt. If your domains dns is manged externally via you registrar then you will need to point them to your Plesk panels IP address.
Once you have that done then you will install the needed docker files via the CLI using Putty or whatever you are happy using.
Then if you run these commands:
docker run --name db -d mongo:3.0 --smallfiles
docker run --name rocketchat --link db -d rocketchat/rocket.chat:latest
mongodb://db:27017/rocketchat
You can test to see if Rocket Chat is working by going to your IP address IE: https://101.101.101.101:3000
Once it has installed you will then need to go back to the Plesk panel and Docker and you can add the values for your sub domain.
Go to your sub domain and add the docker port options and you should then be able to view Rocket Chat via your sub domain.
This is how I had to do it on Plesk.
If you need any more help just ask