Implement cache using docker's --cache-from flag

[databus23]

With docker 1.13 a new build flag was added: --cache-from
This flag deprecated the buildcache tool that we used with docker 1.12

Details: moby/moby#26839

[concourse-bot]

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• #139689311 Implement cache using docker's --cache-from flag

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[databus23]

rebased on master.
This removes a lot of cruft from the image and even makes the caching use-case a tad faster. I would really like to see this in 2.8.0.
ping @vito

[vito]

sweet! thanks.