Retain a fixed number of containers when pruning

Time based container and image retention can have variable space
requirements depending on how often we deploy.

- Only prune stopped containers, retaining the 5 newest
- Then prune dangling images so we only keep images for the retained
containers.

test/cli/prune_test.rb

@@ -10,13 +10,13 @@ class CliPruneTest < CliTestCase
 
   test "images" do
     run_command("images").tap do |output|
-      assert_match /docker image prune --all --force --filter label=service=app --filter until=168h on 1.1.1.\d/, output
+      assert_match /docker image prune --all --force --filter label=service=app --filter dangling=true on 1.1.1.\d/, output
     end
   end
 
   test "containers" do
     run_command("containers").tap do |output|
-      assert_match /docker container prune --force --filter label=service=app --filter until=72h on 1.1.1.\d/, output
+      assert_match /docker ps -q -a --filter label=service=app --filter status=created --filter status=exited --filter status=dead | tail -n +6 | while read container_id; do docker rm $container_id; done on 1.1.1.\d/, output
     end
   end