Port flagged as an error if an build argument does not have a default value

[rcjsuen]

FROM scratch
ARG ZOO_PORT
EXPOSE $ZOO_PORT

Given the above Dockerfile, an error will be reported because we can't resolve the ZOO_PORT variable to a value that satisfies our validation of an EXPOSE instruction. However, given that it is a build argument that the user will pass to Docker, we should just ignore it and not bother trying to validate it.

[rcjsuen]

Fixed in cc4d893 by adopting version 0.0.4 of dockerfile-utils.

Please refer to rcjsuen/dockerfile-utils#3 for more details.