deploy and use docker

[casperdcl]

• auto build & deploy a docker image
• when actions run, use a deployed docker image rather than re-building

Due to the tiny-yet-slow alpine-based Docker image, downloading a pre-built image should be much quicker than re-building it each time this action runs... best of both.

[casperdcl]

Almost-yet-didn't work: https://github.com/casperdcl/test/runs/517690686?check_suite_focus=true#step:2:1