Allow custom subnet to be provided when private network type is dhcp

plugins/providers/docker/action/connect_networks.rb

@@ -55,7 +55,7 @@ module VagrantPlugins
             end
 
             @logger.debug("Connecting network #{network_name} to container guest #{machine.name}")
-            if options[:ip]
+            if options[:ip] && options[:type] != "dhcp"
               if IPAddr.new(options[:ip]).ipv4?
                 network_options[:ip] = options[:ip]
               else

plugins/providers/docker/action/prepare_networks.rb

@@ -85,7 +85,7 @@ module VagrantPlugins
             network_name = root_options[:name]
           end
 
-          if root_options[:type].to_s == "dhcp"
+          if root_options[:type].to_s == "dhcp" && !root_options[:ip]
             network_name = "vagrant_network" if !network_name
             return [network_name, network_options]
           end