Base Image with Variables Resulting in Invalid Reference Format

[SiegeSailor]

I have a private Docker registry that mirrors Docker images, and I have the following in my Dockerfile:

ARG REGISTRY_DOCKER="foo.bar.io/mirror-docker-remote"

# [Stage] Install system-level dependencies and define variables
FROM $REGISTRY_DOCKER/buildpack-deps:22.04@sha256:3084807a4d9b04eea7ec6095e8797e327fb415eddae610218d9df96c4972a339 AS system

ARG REGISTRY_DOCKER

Then, when I try to pin the versions:

user@MacBook foo % dockpin docker pin -f ./docker-context/Dockerfile                                      
Resolving digest of $REGISTRY_DOCKER/buildpack-deps:22.04...
Error: Error response from daemon: invalid reference format
Error response from daemon: invalid reference format

Is it possible for dockpin to resolve variables or just ignore that line?