How to fetch dependencies via HTTP?

I’m building a Docker image to install our site’s RT, configured just the way we like it. As I debug the Docker image, I’m fetching and install RT and its extensions multiple times. I’ve run into a problem where the install is extremely slow, and it’s really slowing the process for me.

My corporate firewall blocks all FTP traffic. But the RT modules try to fetch their dependencies using FTP first, with a 10 minute timeout for each dependency. This means every install is very slow. Is there a way to completely disable FTP and just use HTTP?