Is there an easy way to enable https for the horizon server? If not, how would you recommend to update the docker file to enable https?
Put a loadbalancer, nginx, or apache in front of it. E.g. in amazon you would terminate https on the load balancer typically. You should not deploy your https private key to individual docker containers.
There is also a TLS Cert and TLS Key flag that you can invoke on stellar-core within the docker container. From there, you'd map port
8000 on the docker execution command.