Docker: Move from Slim to Alpine

[hugomd]

The node slim image by itself is 219 MB, then with all of our build steps it jumps to 321 MB in total.

My recommendation would be that we move to using the node 10-alpine image, which starts at 23 MB, and then install git on top of that, rather than installing it via apt as we do in the slim image:

stage-ci/Dockerfile

Line 3 in 408f6a0

RUN apt-get -y update && apt-get -y install git

This should reduce overall size and deployment time significantly.

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