RFC: use docker to speedup CI + run on more environments

[timotheecour]

goal

• speedup CI: no need to keep building koch
• run on more environments, eg netbsd (not support by other CI's at the moment)

CIs supporting docker

• GitHub Actions: should support it, eg https://help.github.com/en/actions/building-actions/dockerfile-support-for-github-actions and https://www.docker.com/blog/first-docker-github-action-is-here/
• azure pipelines: should support it, eg https://docs.microsoft.com/en-us/azure/devops/pipelines/ecosystems/containers/build-image?view=azure-devops
• travis and circleci too

but we should use either azure or github actions, for reasons of higher parallelism among other things

/cc @alaviss in reference to explanation here nim-lang#13907 (comment) I'm wondering whether it'd be better to build a docker image for 32 bit

links

• Tests failing on NetBSD nim-lang/Nim#4794 (comment)
• status-im/nim-docker: Nim docker images used to build Status nim projects
• Docker-Hub-frolvlad/docker-alpine-nim: The smallest Docker image with Nim programming language (~164MB)
• https://hub.docker.com/r/nimlang/nim/

note

• unclear whether the speedup argument is important; looks like building nim from csources is < 1mn

[krytarowski]

Docker is temporarily unsupported on NetBSD. It should change in future, but we are not there yet.

[timotheecour]

docker run -it madworx/netbsd:8.0-x86_64 bash works though (and there may be a way to enable KVM, haven't looked into it) ; the goal would be to run docker on a CI machine (say on a ubuntu machine) to get netbsd as workaround for lack of CI's that support netbsd