Allow Mac/Windows users to use Docker native

plugins/providers/docker/config.rb

@@ -1,5 +1,7 @@
 require "pathname"
 
+require_relative "../../../lib/vagrant/util/platform"
+
 module VagrantPlugins
   module DockerProvider
     class Config < Vagrant.plugin("2", :config)
@@ -202,7 +204,6 @@ def finalize!
         @create_args = [] if @create_args == UNSET_VALUE
         @dockerfile = nil if @dockerfile == UNSET_VALUE
         @env       ||= {}
-        @force_host_vm = false if @force_host_vm == UNSET_VALUE
         @has_ssh    = false if @has_ssh == UNSET_VALUE
         @image      = nil if @image == UNSET_VALUE
         @name       = nil if @name == UNSET_VALUE
@@ -223,6 +224,13 @@ def finalize!
           @host_vm_build_dir_options = nil
         end
 
+        # On non-linux platforms (where there is no native docker), force the
+        # host VM. Other users can optionally disable this by setting the
+        # value explicitly to false in their Vagrantfile.
+        if @force_host_vm == UNSET_VALUE
+          @force_host_vm = !Vagrant::Util::Platform.linux?
+        end
+
         # The machine name must be a symbol
         @vagrant_machine = @vagrant_machine.to_sym if @vagrant_machine
 

test/unit/plugins/providers/docker/config_test.rb

@@ -60,6 +60,7 @@ def valid_defaults
   before do
     # By default lets be Linux for validations
     Vagrant::Util::Platform.stub(linux: true)
+    Vagrant::Util::Platform.stub(linux?: true)
   end
 
   it "should be invalid if both build dir and image are set" do

website/source/docs/docker/configuration.html.md

@@ -50,7 +50,11 @@ General settings:
 
   * `force_host_vm` (boolean) - If true, then a host VM will be spun up
     even if the computer running Vagrant supports Linux containers. This
-    is useful to enforce a consistent environment to run Docker.
+    is useful to enforce a consistent environment to run Docker. This value
+    defaults to "true" on Mac and Windows hosts and defaults to "false" on
+    Linux hosts. Mac/Windows users who choose to use a different Docker
+    provider or opt-in to the native Docker builds can explicitly set this
+    value to false to disable the behavior.
 
   * `has_ssh` (boolean) - If true, then Vagrant will support SSH with
     the container. This allows `vagrant ssh` to work, provisioners, etc.