That is correct, the workaround for this is to set the host name for the container.
--hostname git.subdomain.domain.com
This is needed for the reverse proxy, use your domain name here instead of the external url omnibus varaible.
Since the external url can not be set if you are not using https, this is the work around. When no external url is set, the project clone urls are created from the container hostname. So this is why you have to set the container hostname when using the docker behind a reverse proxy.
My Docker container is picking this up for all the repo addresses and all my urls work behind a nginx reverse proxy now.