slack: Add block color context

[fragoulis]

The PR provides a small fix to the slack message sent by cloud build.

The two commits have a detailed commit message so I will avoid duplication here.

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[wyardley]

@emoryruscus @Aric1088 (just bugging you because of the recent merge / review)- would you be able to get someone to look at this and / or #137?

Thanks to whoever rebuilt the Docker image (finally) as well.

[ericnelson]

Any plans to merge this in and rebuild the docker image ?

[albosse]

Could some please review & build this?

[fayfaychan]

/gcbrun