providers/docker: Implement public_address capability for `vagrant share`

plugins/providers/docker/cap/public_address.rb

@@ -0,0 +1,15 @@
+module VagrantPlugins
+  module DockerProvider
+    module Cap
+      module PublicAddress
+        def self.public_address(machine)
+          return nil if machine.state.id != :running
+
+          ssh_info = machine.ssh_info
+          return nil if !ssh_info
+          ssh_info[:host]
+        end
+      end
+    end
+  end
+end

plugins/providers/docker/plugin.rb

@@ -28,6 +28,11 @@ module VagrantPlugins
         SyncedFolder
       end
 
+      provider_capability("docker", "public_address") do
+        require_relative "cap/public_address"
+        Cap::PublicAddress
+      end
+
       protected
 
       def self.init!