providers/docker: can force a host VM optionally

plugins/providers/docker/config.rb

@@ -22,6 +22,11 @@ module VagrantPlugins
       # @return [Hash]
       attr_accessor :env
 
+      # Force using a proxy VM, even on Linux hosts.
+      #
+      # @return [Boolean]
+      attr_accessor :force_host_vm
+
       # True if the Docker container exposes SSH access. If this is true,
       # then Vagrant can do a bunch more things like setting the hostname,
       # provisioning, etc.
@@ -66,6 +71,7 @@ module VagrantPlugins
         @cmd        = UNSET_VALUE
         @create_args = []
         @env        = {}
+        @force_host_vm = UNSET_VALUE
         @has_ssh    = UNSET_VALUE
         @image      = UNSET_VALUE
         @name       = UNSET_VALUE
@@ -100,6 +106,7 @@ module VagrantPlugins
         @cmd        = [] if @cmd == UNSET_VALUE
         @create_args = [] if @create_args == 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

plugins/providers/docker/provider.rb

@@ -118,9 +118,7 @@ module VagrantPlugins
       # rather than directly on our system. Docker needs to run in a VM
       # when we're not on Linux, or not on a Linux that supports Docker.
       def host_vm?
-        # TODO: It'd be nice to also check if Docker supports the version
-        # of Linux that Vagrant is running on so that we can spin up a VM
-        # on old versions of Linux as well.
+        @machine.provider_config.force_host_vm ||
           !Vagrant::Util::Platform.linux?
       end
 

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

@@ -41,6 +41,7 @@ describe VagrantPlugins::DockerProvider::Config do
     its(:build_dir) { should be_nil }
     its(:cmd) { should eq([]) }
     its(:env) { should eq({}) }
+    its(:force_host_vm) { should be_false }
     its(:image) { should be_nil }
     its(:name) { should be_nil }
     its(:privileged) { should be_false }