Keycloak and Docker - Cannot set two types of URLs
According to the following links, it appears you can use the same DNS for external requests as you would for internal. See these:
http://keycloak.github.io/docs/userguide/keycloak-server/html_single/index.html#d4e4114
You should set the KEYCLOAK_FRONTEND_URL parameter in the Dockerfile or docker-compose.yml (if you use them). In other case your should set this parameter in Keycloak General settings UI.
Eg.:
It is quite tricky because you shouldn't set the real front-end's URL, however you should set the URL which is used by front-end. I have the same problem so you can see some examples in my SO question/answer