Introduce strategy for runner behavior

The runner model is only a class responsible for storing information
now. Based on the configuration it picks a strategy for the runner
management. The Poseidon strategy is already implemented and tested.
The Docker strategy will follow.

spec/lib/runner/strategy/docker_spec.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe Runner::Strategy::Docker do
+  let(:runner_id) { FactoryBot.attributes_for(:runner)[:runner_id] }
+  let(:execution_environment) { FactoryBot.create :ruby }
+  let(:docker) { 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(docker.public_methods).to include(*Runner::DELEGATED_STRATEGY_METHODS)
+    expect(described_class.public_methods).to include(:request_from_management)
+  end
+end