Docker backend

[MisterDA]

This PR implements the Docker backend for OBuilder. It should support running on Windows with Docker for Windows, and on Linux with the usual Docker. This PR contains a batch of prerequisites, and then the backend. I have tested it on Windows and Linux, on basic examples.

• This PR requires some patches to Lwt. Some have been merged to the development branch already.
  • https://github.com/ocsigen/lwt/pull/869
  • https://github.com/ocsigen/lwt/pull/886
  • https://github.com/ocsigen/lwt/pull/898
  • https://github.com/ocsigen/lwt/pull/901
• [ ] Test the OBuilder Docker backend with ocaml-ci, opam-health-check, and opam-repo-ci.

[talex5]

Thinking on about this... the Build module creates a builder from a store and a sandbox. That will work on Linux (runc+btrfs), mac (users+zfs) and probably on FreeBSD (jails+zfs). But it doesn't work with Docker, because Docker already combines a store and a sandbox. So probably this should be an alternative to the build.ml module instead.

[MisterDA]

Thanks for the review, I agree with your conclusion and will refactor the code to merge the Docker sandbox, Docker_storage, the Docker-related code in build.ml, and maybe the code to talk to the client in the Docker module into a single Docker_build module.

A note regarding the use of the Docker Engine API: I wanted to replace the process calls to the docker client with the Docker Engine API, talking directly to dockerd with sockets or pipes. However

• ocaml-docker supports API 1.29 and would need to be ported to Windows too;
• openapi-generator, a tool that generates bindings from a standardized yaml description of the API doesn't support the latest version of the standard and generates invalid OCaml code (see https://github.com/OpenAPITools/openapi-generator/issues/9787).

I'm putting this on hold for now, and I'll stay with the docker client until openapi-generator is fixed.

[MisterDA]

Now depends on

• 116

• 117

[MisterDA]

I lost the race to merge before macOS. The rebase wasn't easy, I've probably broken one or two new things in my code.