Pull latest image tag, so we can identity it

`docker image ls` doesn't tell us what the latest deployed image is (e.g
if we've rolled back). Pull the latest image tag through to the server
so we can use it instead.

test/cli/app_test.rb

@@ -28,9 +28,8 @@ class CliAppTest < CliTestCase
       .returns([ :docker, :run ])
 
     run_command("boot").tap do |output|
-      assert_match "Rebooting container with same version 999 already deployed", output # Can't start what's already running
-      assert_match "docker container ls --all --filter name=app-999 --quiet | xargs docker container rm", output # Stop old running
-      assert_match "docker container ls --all --filter name=app-999 --quiet | xargs docker container rm", output # Remove old container
+      assert_match "Rebooting container with same version latest already deployed", output # Can't start what's already running
+      assert_match "docker container ls --all --filter name=app-latest --quiet | xargs docker container rm", output # Remove old container
       assert_match "docker run", output # Start new container
     end
   ensure

test/cli/build_test.rb

@@ -29,7 +29,7 @@ class CliBuildTest < CliTestCase
   test "pull" do
     run_command("pull").tap do |output|
       assert_match /docker image rm --force dhh\/app:999/, output
-      assert_match /docker pull dhh\/app:999/, output
+      assert_match /docker pull dhh\/app:latest/, output
     end
   end
 

test/commands/app_test.rb

@@ -189,12 +189,6 @@ class CommandsAppTest < ActiveSupport::TestCase
       new_command.current_running_version.join(" ")
   end
 
-  test "most_recent_version_from_available_images" do
-    assert_equal \
-      "docker image ls --format \"{{.Tag}}\" dhh/app | head -n 1",
-      new_command.most_recent_version_from_available_images.join(" ")
-  end
-
   test "list_containers" do
     assert_equal \
       "docker container ls --all --filter label=service=app",