Fix aggressive prune breaking rollback

In the image prune command --all overrides --dangling=true. This removes
the image git sha image tag for the latest image which prevented
us from rolling back to it.

I've updated the integration test to now test deploy, redeploy and
rollback.

test/commands/prune_test.rb

@@ -10,7 +10,7 @@ class CommandsPruneTest < ActiveSupport::TestCase
 
   test "images" do
     assert_equal \
-      "docker image prune --all --force --filter label=service=app --filter dangling=true",
+      "docker image prune --force --filter label=service=app --filter dangling=true",
       new_command.images.join(" ")
   end