Windows support / Docker backend

[MisterDA]

This is a tracking issue to cover getting OBuilder working with Windows and introducing support for Windows builds into ocaml-ci and opam-repo-ci.

The Idea (as it relates to OBuilder)

The two core components of OBuilder are the Sandbox and the Store, the initial Windows support will use Docker for Windows to provide both. The Sandbox is provided by a running Windows container using either HyperV or Process isolation, and the store is using docker images.

Incomplete list of tasks:

• Docker & Windows prerequisites Docker & Windows prerequisites #116
  (merge-me) Will possibly add more patches.
• Add support for Docker/Windows spec Add support for Docker/Windows spec #117
  (merge-me)
• Docker backend Docker backend #75
  Needs to debug OBuilder caches implemented as Docker volumes.
• Add ocluster support for Windows Hook Obuilder Docker backend ocluster#143
• Support Windows builds in ocaml-ci
  Waiting on opam-2.2 and OCaml compiler packages for Windows.
• Support Windows builds in opam-repo-ci
  Waiting on opam-2.2 and OCaml compiler packages for Windows.
  Experimental branch using OCaml for Windows at [WIP] Experimental Windows support opam-repo-ci#218
• Support Windows builds in opam-health-check
  Waiting on opam-2.2 and OCaml compiler packages for Windows.
  Experimental branch using OCaml for Windows at Test OCaml for Windows opam-health-check#32.

Future work

• Revisit using buildkit on Windows https://github.com/moby/buildkit
  Sadly not available on Windows: As a Developer I want to be able to build Windows Containers with BuildKit microsoft/Windows-Containers#34.
• Investigate using HCS and runHCS tooling https://learn.microsoft.com/en-us/virtualization/windowscontainers/deploy-containers/containerd.
• Investigate using zfs on Windows or NTFS native snapshot support as the Store.