Make the concurrent setup prevention opt-in

[vovimayhem]

#26 allowed to skip the concurrent setup lock, but the default behavior should be not to lock the setup at all.

Let's use an optional ON_CONTAINER_PREVENT_CONCURRENT_SETUP environment variable that prevents the concurrent setup instead.

Also, let's rename the on_setup_lock_acquired method with to_setup_container so the development entrypoint scripts look like this:

require 'on_container/dev/rails'

set_given_or_default_command

if command_requires_setup?
  to_setup_container do
    # Stuff to setup the container...
  end
end