Fix bad concorse task docker image reference
Fix bad concorse task docker image reference

Need a tag property rather than using the image:tag syntax

[#156582099](https://www.pivotaltracker.com/story/show/156582099)

Co-authored-by: Danny Berger <8efccf38e9d8f361e1973ad644e1b1e50ad8ecfe@pivotal.io>

cloudfoundry/gosigar,cloudfoundry/gosigar