Docker backend added.
Fixed declarated & not used.
Fixed stop container.
Start workers on runner start.
Copy stderr to stdout if can't start container.
Close streams on finish.
Fixed container initialization.
Merge branch 'runner-refactoring'

* runner-refactoring:
  Fixed container initialization.
  Close streams on finish.
  Copy stderr to stdout if can't start container.
  Start workers on runner start.
  Fixed stop container.
  Fixed declarated & not used.
  Docker backend added.