providers/docker: retry container ssh command [GH-3763]

CHANGELOG.md

@@ -14,6 +14,8 @@ BUG FIXES:
       crash. [GH-3827]
   - providers/docker: Never do graceful shutdown, always use
       `docker stop`. [GH-3798]
+  - providers/docker: Better error messaging when SSH is not ready
+      direct to container. [GH-3763]
   - provisioners/puppet: Fix setting facter vars with Windows
       guests. [GH-3776]
   - guests/darwin: Respect mount options for NFS. [GH-3791]

plugins/providers/docker/communicator.rb

@@ -25,7 +25,9 @@ module VagrantPlugins
         return false if !@host_vm.communicate.ready?
 
         # We're ready if we can establish an SSH connection to the container
-        @host_vm.communicate.test("#{container_ssh_command} exit")
+        command = container_ssh_command
+        return false if !command
+        @host_vm.communicate.test("#{command} exit")
       end
 
       def download(from, to)
@@ -131,6 +133,7 @@ module VagrantPlugins
       def container_ssh_command
         # Get the container's SSH info
         info = @machine.ssh_info
+        return nil if !info
         info[:port] ||= 22
 
         # Make sure our private keys are synced over to the host VM