Remove usage of DockerClient from execution_environments_controller.rb

lib/runner/strategy.rb

@@ -9,6 +9,10 @@ class Runner::Strategy
     raise NotImplementedError
   end
 
+  def self.available_images
+    raise NotImplementedError
+  end
+
   def self.sync_environment(_environment)
     raise NotImplementedError
   end

lib/runner/strategy/docker_container_pool.rb

@@ -8,6 +8,13 @@ class Runner::Strategy::DockerContainerPool < Runner::Strategy
     @config ||= CodeOcean::Config.new(:docker).read(erb: true)
   end
 
+  def self.available_images
+    DockerClient.check_availability!
+    DockerClient.image_tags
+  rescue DockerClient::Error => e
+    raise Runner::Error::InternalServerError.new(e.message)
+  end
+
   def self.sync_environment(_environment)
     # There is no dedicated sync mechanism yet
     true

lib/runner/strategy/poseidon.rb

@@ -13,6 +13,13 @@ class Runner::Strategy::Poseidon < Runner::Strategy
     @config ||= CodeOcean::Config.new(:code_ocean).read[:runner_management] || {}
   end
 
+  def self.available_images
+    # Images are pulled when needed for a new execution environment
+    # and cleaned up automatically if no longer in use.
+    # Hence, there is no additional image that we need to return
+    []
+  end
+
   def self.headers
     @headers ||= {'Content-Type' => 'application/json', 'Poseidon-Token' => config[:token]}
   end