macOS support
A frequent request is to support macOS for composing images. apk-tools itself has been ported to macOS as part of apk-tools v3, so it should be possible to get an apk on mac once #234 is resolved.
However, we need to have an answer for the /bin/busybox --install -s step that gets done in the image. That is the other main blocker for getting image composition running on macOS.
A decent apko build container would help in the short-term as well.
The apko container is in a pretty good place now. We should add some more docs on using the image (and similar for melange).
Added some docs on using the container image to #311