Change example docker command so it can be restarted

[siyu6974]

Added -p option for mkdir so that when bringing back the container with docker start <container> it wouldn't fail.

PR inspired by a SO question: https://stackoverflow.com/questions/53887608/how-re-run-the-continumio-anaconda3-docker-image/53956270#53956270

[estuardolh]

Thanks @siyu6974

Someone to merge this? 🙁 It solves my issue also.

[dbast]

thanks. this got also done by #236 which includes more required updates to the example command.