providers/docker: allow multiple links to same backend (different aliases).

plugins/providers/docker/action/create.rb

@@ -96,10 +96,10 @@ module VagrantPlugins
           image = @env[:create_image]
           image ||= @provider_config.image
 
-          links = {}
+          links = []
           @provider_config._links.each do |link|
             parts = link.split(":", 2)
-            links[parts[0]] = parts[1]
+            links << parts
           end
 
           {

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

@@ -17,7 +17,7 @@ describe VagrantPlugins::DockerProvider::Driver do
       ports:      '8080:80',
       volumes:    '/host/path:guest/path',
       detach:     true,
-      links:      {janis: 'joplin'},
+      links:      [[:janis, 'joplin'], [:janis, 'janis']],
       env:        {key: 'value'},
       name:       cid,
       hostname:   'jimi-hendrix',
@@ -43,7 +43,9 @@ describe VagrantPlugins::DockerProvider::Driver do
     end
 
     it 'links containers' do
-      expect(cmd_executed).to match(/--link #{params[:links].to_a.flatten.join(':')} .+ #{Regexp.escape params[:image]}/)
+      params[:links].each do |link|
+        expect(cmd_executed).to match(/--link #{link.join(':')} .+ #{Regexp.escape params[:image]}/)
+      end
     end
 
     it 'sets environmental variables' do