providers/docker: vagrant_machine setting works proprely

plugins/providers/docker/config.rb

@@ -139,6 +139,9 @@ module VagrantPlugins
         @vagrant_machine = nil if @vagrant_machine == UNSET_VALUE
         @vagrant_vagrantfile = nil if @vagrant_vagrantfile == UNSET_VALUE
 
+        # The machine name must be a symbol
+        @vagrant_machine = @vagrant_machine.to_sym if @vagrant_machine
+
         @expose.uniq!
       end
 

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

@@ -202,6 +202,17 @@ describe VagrantPlugins::DockerProvider::Config do
     end
   end
 
+  describe "#vagrant_machine" do
+    before { valid_defaults }
+
+    it "should convert to a symbol" do
+      subject.vagrant_machine = "foo"
+      subject.finalize!
+      assert_valid
+      expect(subject.vagrant_machine).to eq(:foo)
+    end
+  end
+
   describe "#vagrant_vagrantfile" do
     before { valid_defaults }