providers/docker: notify when proxying SSH

plugins/providers/docker/action/prepare_ssh.rb

@@ -10,6 +10,9 @@ module VagrantPlugins
           # If we aren't using a host VM, then don't worry about it
           return @app.call(env) if !env[:machine].provider.host_vm?
 
+          env[:machine].ui.output(I18n.t(
+            "docker_provider.ssh_through_host_vm"))
+
           # Get the container's SSH info
           info = env[:machine].ssh_info
           info[:port] ||= 22

templates/locales/providers_docker.yml

@@ -19,6 +19,9 @@ en:
       Container not created. Skipping.
     not_docker_provider: |-
       Not backed by Docker provider. Skipping.
+    ssh_through_host_vm: |-
+      SSH will be proxied through the Docker virtual machine since we're
+      not running Docker natively. This is just a notice, and not an error.
 
     messages:
       destroying: |-