Fix parsing of build args in build-image template

    . build.env

  - |

    : buildctl build ...

    opts() { test $# -eq 0 || printf "--opt build-arg:%s" "$@"; }

    opts() { test $# -eq 0 || printf " --opt\0build-arg:%s\0" "$@"; }

    build()

    {

      opts "$@" | xargs buildctl build \

      opts "$@" | xargs -0 buildctl build \

      --frontend dockerfile.v0 \

      --local context=. \

      --local dockerfile=. \