Module sends http url, instead of https url, to CAS when behind reverse proxy

I'm running Drupal 8 behind an Apache reverse proxy. It's running inside a docker container. So my https configuration is done by the proxy. Not in the container. This seems to make the CAS module think it should set the service URL to http, when it really should be https.

So, the url the CAS module sends looks like: https://testcas.example.org/cas/login?service=http%3A//testdrupal.exampl...

It should look like: https://testcas.example.org/cas/login?service=https%3A//testdrupal.examp...

Would it make sense to add an override option to the CAS module's config that would let me tell it to use the correct service url?

In the meantime, do you have any ideas on how I can work around this?

My first thought is to try setting Drupal's base_url in settings.php. So I'll go see if that works.

Thanks in advance. :)