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.

lib/runner/strategy/docker.rb

@@ -0,0 +1,3 @@
+# frozen_string_literal: true
+
+class Runner::Strategy::Docker < Runner::Strategy; end