Exposing multiple docker containers running locally

I am creating a SaaS project that lets end-users run and access a dashboard as a web app which is a Docker container, which means every user has his own dashboard which is a running docker container, and I would like them to access their servers/containers using my domain (HTTP) as follow: user1: subdomain.mydomain.com/user1app, user2: subdomain.mydomain.com/user1app, etc.

Currently, I am using LocalTunnel however it is not stable and requires me to use 1 subdomain for every user, for example: user1.mydomain.com, user2.mydomain.com, etc. but what if we scale and get more users? I need a dynamic and automatic way to create users custom URL link to expose their running docker containers and give them access such as, subdomain.mydomain.com/user123, subdomain.mydomain.com/user456, etc.

I tried to use ngrok, however, it is limited in many ways e.g. 40-requests/minute limit, and not free. Thanks