Don't pass -z to tar

[fatalbanana]

AFAICT docker save will produce an uncompressed tar archive.

This prevented successful run of analyze-local-images:

2015/11/23 13:59:30 - Could not save image: 
gzip: stdin: not in gzip format
tar: Child died with signal 13
tar: Error is not recoverable: exiting now

The patch fixed it. :)

If docker save will in fact produce compressed output?- & one's implementation of tar is not capable of self-identifying compression, then perhaps this would break things. :\

[Quentin-M]

You are right ! Good catch, my tar implementation is capable of skipping -z the archive is not actually gzip'd.

Can you just edit your commit message to make it more standard ? Something like contrib: Don't pass -z to tar in analyze-local-images ?

Thanks for your contribution.