Replace example grep pattern by a working pattern

[DeviaVir]

The problem I was running into, with the spotify docker from dockerhub, was that I had a pattern set in my docker exclude file for (for example) ruby:
ruby:[^ ]\+
But the ruby images were getting cleaned up regardless.

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.

[ownaginatious]

Are you sure you were using the same version of Docker on all machines? There is currently a bug where everything gets cleaned up by docker-gc regardless of exclusion/inclusion patterns for Docker 1.10+ due to image signatures being now prefixed with sha256. Looks like there are a couple fixes about a month old now that haven't been accepted yet.

[DeviaVir]

@ownaginatious yep! Was using docker 1.9, pretty sure this has something to do with grep on Alpine

[mattnworb]

@DeviaVir thanks for the contribution! not sure why the original contribution chose to use a less obvious example compared to this one.