ensure the use of GNU tar when creating builds on OSX

[kyleburton]

tar archives (such as the ones created for releases of bake, used by homebrew) created on OS X will emit errors when the archive is extracted on Linux machines:

tar: Ignoring unknown extended header keyword 'SCHILY.dev'
tar: Ignoring unknown extended header keyword 'SCHILY.ino'
tar: Ignoring unknown extended header keyword 'SCHILY.nlink'
tar: Ignoring unknown extended header keyword 'SCHILY.dev'
tar: Ignoring unknown extended header keyword 'SCHILY.ino'
tar: Ignoring unknown extended header keyword 'SCHILY.nlink'
tar: Ignoring unknown extended header keyword 'SCHILY.dev'
tar: Ignoring unknown extended header keyword 'SCHILY.ino'
tar: Ignoring unknown extended header keyword 'SCHILY.nlink'
tar: Ignoring unknown extended header keyword 'SCHILY.dev'
tar: Ignoring unknown extended header keyword 'SCHILY.ino'
tar: Ignoring unknown extended header keyword 'SCHILY.nlink'
tar: Ignoring unknown extended header keyword 'SCHILY.dev'
tar: Ignoring unknown extended header keyword 'SCHILY.ino'
tar: Ignoring unknown extended header keyword 'SCHILY.nlink'
tar: Ignoring unknown extended header keyword 'SCHILY.dev'
tar: Ignoring unknown extended header keyword 'SCHILY.ino'
tar: Ignoring unknown extended header keyword 'SCHILY.nlink'

This is a known issue (https://superuser.com/questions/318809/linux-os-x-tar-incompatibility-tarballs-created-on-os-x-give-errors-when-unt) and can, perhaps, be solved by either ensuring GNU tar is installed, or using a small docker image to execute the release & build process.