Do not allow both host and hosts for proxy configuration

lib/kamal/configuration/docs/proxy.yml

@@ -31,7 +31,7 @@ proxy:
   # Hosts
   #
   # The hosts that will be used to serve the app. The proxy will only route requests
-  # to this host to your app.
+  # to these hosts to your app.
   #
   # If no hosts are set, then all requests will be forwarded, except for matching
   # requests for other apps deployed on that server that do have a host set.

lib/kamal/configuration/validator/proxy.rb

@@ -6,6 +6,10 @@ class Kamal::Configuration::Validator::Proxy < Kamal::Configuration::Validator
       if config["host"].blank? && config["hosts"].blank? && config["ssl"]
         error "Must set a host to enable automatic SSL"
       end
+
+      if config["host"].present? && config["hosts"].present?
+        error "Must use either 'host' or 'hosts', not both"
+      end
     end
   end
 end