Improve build and test experience for developers using windows

[mkmik]

Developers having only windows machines and who are not repository owners (or even if they are, they are using personal forks of the v7 repo) cannot currently easily check whether their changes break a build.

make docker, (with boot2docker) is an easy way to reproduce our build, but we need a couple of tweaks in the makefile to make it work seamlessly on windows.

[mkmik]

Circleci offers an experimental feature which enables builds coming from external forks.

However, it's an insecure option because it exposes all our project's conf, including environment variables containing API keys. We currently only use the coveralls API (actually we do it only in fossa at the moment, but we should do it on v7 as well), so it's not a big deal, but still...

It's not strictly necessary though, given that we could also just improve the build and experience for windows developers, which will also have a positive side effect of including the ecma report file in the PR.

[cpq]

Docker way sgtm

[mkmik]

• [x] MSVC build with docker is already implemented.
• [ ] mingw cross target inside docker vm
• [ ] native mingw target