Add a sleep-loop to get the Docker daemon's tcp socket - the pid file is...

...n't ready the first time we ask for it on one slower box I have

and in the process, it was easier to return the error and use it...
For future reference, it is a good habit to put `regexp.MustCompile` calls outside of all functions when they are static, as they can then be processed once at startup instead of upon each function call, since regexp compilation can be an expensive operation (not that this code is time-critical)... just FYI.