Docker: Support ARG FROM

[hfhbd]

Package ecosystem
docker
Package manager version
"Dockerfile"
Language version
docker
Manifest location and content prior to update

ARG HUB=docker.io
FROM ${HUB}/docker:20.10.7-dind

dependabot.yml content

version: 2
updates:
  - package-ecosystem: "docker"
    directory: "/"
    schedule:
      interval: "daily"

Updated dependency

ARG HUB=docker.io
FROM ${HUB}/docker:20.10.12-dind

What you expected to see, versus what you actually saw
Actually, this line does not support ARGs parsing including replacing:

dependabot-core/docker/lib/dependabot/docker/file_parser.rb

Line 42 in b7ad648

next unless FROM_LINE.match?(line)

Dependabot should put all ARGs in a map and replace the values before executing the regex.
This syntax is valid: https://docs.docker.com/engine/reference/builder/#understand-how-arg-and-from-interact

Native package manager behavior
n/a
Images of the diff or a link to the PR, issue or logs
n/a, because Dependabot does not detect this line as valid FROM syntax, so no dependency was detected.
🕹 Bonus points: Smallest manifest that reproduces the issue

[jeffwidman]

I'm temp re-opening due to the reversion (explained in #4837) and then closing as a dupe of #2057