Great post Sascha, I am sure it will help a lot of people!
I don’t have a Phoenix app in production yet but for anyone using HAProxy, these lines on the frontend may also be of help:
reqadd X-Forwarded-Proto:\ https if { ssl_fc }
redirect scheme https if ssl_redirect_hosts !{ ssl_fc }
redirect scheme https code 301 if !{ ssl_fc }
In conjunction with configuring plug to handle X-Forwarded-* headers as per Sascha’s post.