Incorrect ARG warning in multistage dockerfile

[ngan]

Issue details

If you have an ARG that you'd like to be shared for all stages, the convention is to do the following:

# This is global.
ARG FOO=/path/to/something

FROM alpine:latest as base1
# Call `ARG` with just the key to load the variable from global.
ARG FOO
COPY $FOO/bar .
WORKDIR $FOO

The linter will then say:

COPY contains undefined ARG or ENV
WORKDIR path $FOO must be absolute

Steps to reproduce/test case

See description above

Affected version(s)

master