Add strategy for DockerContainerPool

In order to provide an alternative to Poseidon, a strategy for the
DockerContainerPool is added that is used by the runner model.

Co-authored-by: Sebastian Serth <Sebastian.Serth@hpi.de>

spec/lib/runner/strategy/docker_container_pool_spec.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe Runner::Strategy::DockerContainerPool do
+  let(:runner_id) { FactoryBot.attributes_for(:runner)[:runner_id] }
+  let(:execution_environment) { FactoryBot.create :ruby }
+  let(:container_pool) { described_class.new(runner_id, execution_environment) }
+
+  # TODO: add tests for these methods when implemented
+  it 'defines all methods all runner management strategies must define' do
+    expect(container_pool.public_methods).to include(:destroy_at_management, :copy_files, :attach_to_execution)
+    expect(described_class.public_methods).to include(:request_from_management)
+  end
+end