providers/docker: public_address reports host machine addr [GH-4342]

CHANGELOG.md

@@ -16,6 +16,8 @@ BUG FIXES:
       capture the final image. [GH-4598]
   - providers/docker: NFS synced folders now work. [GH-4344]
   - providers/docker: Read the created container ID more robustly.
+  - providers/docker: `vagrant share` uses correct IP of proxy VM if it
+      exists. [GH-4342]
   - providers/virtualbox: Show a human-friendly error if VirtualBox didn't
       clean up an existing VM. [GH-4681]
   - provisioners/docker: Search for docker binary in multiple places. [GH-4580]

plugins/providers/docker/cap/public_address.rb

@@ -5,6 +5,14 @@ module VagrantPlugins
         def self.public_address(machine)
           return nil if machine.state.id != :running
 
+          # If we're using a host VM, then return the IP of that
+          # rather than of our own machine.
+          if machine.provider.host_vm?
+            host_machine = machine.provider.host_vm
+            return nil if !host_machine.provider.capability?(:public_address)
+            return host_machine.provider.capability(:public_address)
+          end
+
           ssh_info = machine.ssh_info
           return nil if !ssh_info
           ssh_info[:host]