Support the new `--unpack` flag

[rcjsuen]

# syntax=docker/dockerfile:1
FROM debian
ADD --unpack \
	https://github.com/asdf-vm/asdf/releases/download/v0.18.0/asdf-v0.18.0-linux-amd64.tar.gz \
	/usr/local/bin/

Line: 3
ADD --unpack \
    ^^^^^^^^
Error: Unknown flag: unpack

[tcyrus]

Based on the docs, the --unpack flag should use the boolean validator

dockerfile-utils/src/dockerValidator.ts

Line 705 in c6a0f2b

} else if (name === "link" || name === "keep-git-dir") {

} else if (name === "link" || name === "keep-git-dir" || name === "unpack") {