Add environment variable to activate/deactivate the use of https

[jrchatruc]

When deploying to k3s I ran into trouble making the ingress (which acts as an internal load balancer for the app in k3s) work. I created the cert and key files but when trying to access the site got a Bad Gateway error, so as a temporary solution I changed prod.config to use http. It would be nice to have an env variable controlling this.