Homebrew Formula

I've started looking at what it'll take to get vpnkit into Homebrew-core or at minimum in a tap.

• I think the main blocking issue is the legacy OPAM version that appears to be required. Looking at the CircleCI config the project is using 1.2.2_4: https://github.com/moby/vpnkit/blob/master/.circleci/config.yml#L19.

• The makefiles assume they're running from within a git repo. A workaround is to have the brew formula fetch the git repo, but more ideal is fetching the released source tar file from GitHub.

• The vpnkit tagged version 0.3.0 appears somewhat dated with a number of important fixes since then. It looks like vpnkit in Docker Desktop on macOS is compiled from ea9dbeaf887f5dad8391f4a34d127501fb6bbf64. Would it make sense to tag and release that as 0.4.0 since its already widely deployed?

• Progress so far: https://github.com/bensallen/homebrew-vpnkit/blob/master/vpnkit.rb

Start with .circleci/config.yml, the makefiles, and the existing vpnkit.rb formula linked in the issue. Check how the current OPAM version, repository assumptions, and tagged 0.3.0 source affect packaging; done means the proposed Homebrew formula can build from an appropriate released source and the release-version question is resolved.