coder_app shall allow docker network scoped urls other than localhost

Use case:
I want to add a coder_app which opens another container's url instead of being limited to `http:/:localhost:[PORT]/[SUBPATH]` :
(Of course, both containers are part of the same docker network, and the url resolve correctly when using `curl`.)

example:
```tf
resource "coder_app" "auth_phpmyadmin" {
agent_id = coder_agent.workspace.id
slug = "pma-auth"
icon = "https://www.phpmyadmin.net/static/images/logo.png"
url = "http://other-container-phpmyadmin:80/"
subdomain = true
share = "authenticated"

}
```