This image is to build msgpack.gem for mingw32 for both of x86 and x64.
run this image, then got 2 gem binary in
docker run -it --rm -v `pwd`/pkg:/home/ubuntu/msgpack-ruby/pkg msgpack/msgpack-rubygem-crosscompile
-v option is required to specify where binary gems are written.
Optional environment variables below are available to specify what to build.
MSGPACK_REPO: repository where msgpack-ruby will be fetched from
BUILD_BRANCH: branch name to build (master)
BUILD_POSITION: position or hash to build (HEAD)