Evaluate variable default values in determining if blank

[duckbrain]

If I create a Dockerfile with the following contents

FROM ${VAR:-ignore}

I get a diagnostic error:

dockerfile-utils: base name (${VAR:-ignore}) should not be blank [51]

The :- syntax provides a default value so if $VAR is blank, ignore is used instead, so there's no way for the image name to be blank.

[rcjsuen]

@duckbrain Thank you for bringing this to our attention. I have opened rcjsuen/dockerfile-utils#126 to track this.

[rcjsuen]

@duckbrain Thanks again for the bug report. The latest 0.13.0 release of the language server should not flag this as an error anymore.