provisoiners/docker: only add user to group if not in it [GH-2838]

CHANGELOG.md

@@ -68,6 +68,8 @@ BUG FIXES:
     retryable to avoid spurious VirtualBox errors. [GH-2831]
   - providers/virtualbox: Config validation catches invalid keys. [GH-2843]
   - provisioners/chef: When chowning folders, don't follow symlinks.
+  - provisioners/docker: Only add SSH user to docker group if the user
+    isn't already in it. [GH-2838]
   - provisioners/salt: Options can now set install type and install args.
     [GH-2766]
   - synced\_folders/nfs: NFS entries are pruned on every `vagrant up`,

plugins/provisioners/docker/cap/linux/docker_configure_vagrant_user.rb

@@ -4,7 +4,13 @@ module VagrantPlugins
       module Linux
         module DockerConfigureVagrantUser
           def self.docker_configure_vagrant_user(machine)
-            machine.communicate.sudo("usermod -a -G docker #{machine.config.ssh.username || "vagrant"}")
+            ssh_info = machine.ssh_info
+
+            machine.communicate.tap do |comm|
+              if !comm.test("id -Gn | grep docker")
+                comm.sudo("usermod -a -G docker #{ssh_info[:username]}")
+              end
+            end
           end
         end
       end