How to change the port docker is using? - help

I’m playing around with setting up discourse (forum software). I installed docker under debian jessie and run discourse in a container.
Until this step everything works fine.
To let other webpages run on the same machine then discourse I installed nginix while following this instruction: https://meta.discourse.org/t/running-other-websites-on-the-same-machine-as-discourse/17247

But for some reasons docker is then still running on port 80 same as nginx does.

Question: How do I change the port docker is using?
How do I change the port nginx is using?

…hope someone can help me out. My technical experience is quite small…