Port gets flagged as an error if it refers to a build argument without a default value

[rcjsuen]

FROM scratch
ARG ZOO_PORT
EXPOSE $ZOO_PORT

> dockerfile-utils lint Dockerfile
Line: 3
EXPOSE $ZOO_PORT
       ^^^^^^^^^
Error: Invalid containerPort: $ZOO_PORT

The validator is flagging an error because the variable can't be resolved to a value that makes sense for an EXPOSE instruction (as it resolves to nothing). However, as the value will be provided by the user when the Docker image is built, the argument should just be outright ignored by the validator as we should assume that the value provided by the user is valid.