Replace example grep pattern by a working pattern
Replace example grep pattern by a working pattern

It looks like an alpine kind of thing, I tested the code on a few different places (Mac, Debian) where it worked as you'd expect. However, when using Alpine Linux, `[^ ]\+` did not return any IDs and cleaned up images I didn't want it to clean up.

spotify/docker-gc,matt-deboer/docker-gc,Financial-Times/docker-gc