Unexpanded ARG variable causes incorrect warning about an invalid reference format

[rcjsuen]

It is possible to use ARG variables in a FROM so they need to be expanded before we try to validate it.

ARG version=latest
FROM alpine:$version

$ docker build .
Sending build context to Docker daemon  1.695MB
Step 1/2 : ARG version=latest
Step 2/2 : FROM alpine:$version
latest: Pulling from library/alpine
ff3a5c916c92: Pull complete
Digest: sha256:7df6db5aa61ae9480f52f0b3a06a140ab98d427f86d8d5de0bedab9b8df6b1c0
Status: Downloaded newer image for alpine:latest
 ---> 3fd9065eaf02
Successfully built 3fd9065eaf02

$ dockerfile-utils lint Dockerfile.1
Line: 2
FROM alpine:$version
            ^^^^^^^^
Error: invalid reference format

Here are some other Dockerfiles that are also valid.

ARG version=atest
FROM alpine:l$version

ARG atest=atest
FROM alpine:l$atest

ARG DIGEST=sha256:7df6db5aa61ae9480f52f0b3a06a140ab98d427f86d8d5de0bedab9b8df6b1c0
FROM alpine@$DIGEST