add script for updating `Dockerfile.d/SHA256SUMS.d`

[AkihiroSuda]

Suggested by @ktock: #147 (review)

For making the contribution easier, we'll want helper scripts something like this ( https://github.com/kubernetes-sigs/kind/blob/8fe8b962521db017e3ffb22fc410f521dd8bc154/images/base/update-shasums.sh )

[AkihiroSuda]

We will eventually use Dockerfile.pin

• dockerfile: support Dockerfile.pin for pinning sources moby/buildkit#2816