I have a bit of experience with running Dockers in AWS and I assume you are using the mendix-docker-buildpack to create the Dockers. There are three components in our setup:
I believe you never contact the Mendix server directly. At most, you contact nginx, which redirects the call to Mendix. You connect to nginx on port 443 (standard voor HTTPS). If you really need to alter the port, I believe you need to change the configuration of your load balancer and nginx.
Having said that, why would you need a specific port to connect to a SOAP service?