fix permission problem with local bind mount (#1466)

[tkrah]

npm run sanity-test-docker
npm run smoke-test-docker

both are able to run now without permission problems.

[tkrah]

I did not catch that problem initially, because I am already running the backstopjs container with my own user via "--user" command because root owned where a mess ;).
@garris I am not that proficient with all those docker shortcuts you may have created - please have a look if some other command may need that user arg too.

Windows users may need to customize the template and switch to "--user %USERNAME%" - that may be something for you @garris to integrate if you want to ;).

[garris]

LGTM

Thanks @tkrah!