Ignore predefined platform ARG variables if they are used as a base image

[rcjsuen]

FROM alpine as amd64
FROM alpine as arm
FROM ${TARGETARCH}

Given a Dockerfile like the above, it is possible to build the Docker image but we flag it as an error because we cannot resolve ${TARGETARCH}. As it is one of the prepopulated variables, we should allow it to go through since an appropriate build stage may have been defined by the user.

$ dockerfile-utils lint
Line: 3
FROM ${TARGETARCH}
     ^^^^^^^^^^^^^
Error: base name (${TARGETARCH}) should not be blank