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.

lib/mrsk/commands/prune.rb

@@ -2,11 +2,19 @@ require "active_support/duration"
 require "active_support/core_ext/numeric/time"
 
 class Mrsk::Commands::Prune < Mrsk::Commands::Base
-  def images(until_hours: 7.days.in_hours.to_i)
-    docker :image, :prune, "--all", "--force", "--filter", "label=service=#{config.service}", "--filter", "until=#{until_hours}h"
+  def images
+    docker :image, :prune, "--all", "--force", "--filter", "label=service=#{config.service}", "--filter", "dangling=true"
   end
 
-  def containers(until_hours: 3.days.in_hours.to_i)
-    docker :container, :prune, "--force", "--filter", "label=service=#{config.service}", "--filter", "until=#{until_hours}h"
+  def containers(keep_last: 5)
+    pipe \
+      docker(:ps, "-q", "-a", "--filter", "label=service=#{config.service}", *stopped_containers_filters),
+      "tail -n +#{keep_last + 1}",
+      "while read container_id; do docker rm $container_id; done"
   end
+
+  private
+    def stopped_containers_filters
+      [ "created", "exited", "dead" ].flat_map { |status| ["--filter", "status=#{status}"] }
+    end
 end