Improved API

[kit-ty-kate]

Goal:

• Get rid of the calls to docker in obuilder for FreeBSD (Add support for FreeBSD ocurrent/obuilder#120)
• Have a high-level API (that fits obuilder and opam-health-check use-case)
• Have a low-level API (for debug)

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Hurdles:

• Currently http-lwt-client is bugged and can't be used (http-lwt-client does not make use of lwt when downloading something robur-coop/http-lwt-client#12)