Erroneous "failed to process string" error and syntax highlighting for Dockerfile parameter substitution modifier `-`

I'm getting an erroneous error message for the following string:

"failed to process "${HTTP_PROXY-http://corporate-proxy.com:8080}": unsupported modifier (/) in substitution dockerfile-utils(44)

This string replacement is in fact not an error, - is recognized as a valid environment replacement modifier and / is interpreted as the literal character, not a substitution modifier, when performing the substitution (see: https://github.com/docker/docs/issues/23165).

It appears that this is also breaking syntax highlighting.

Tested and confirmed locally:

# Dockerfile
ARG HTTP_PROXY
ENV http_proxy=${HTTP_PROXY-http://corporate-proxy.com:8080}
RUN echo $http_proxy

# No build arg
$ docker build -f Dockerfile .
> http://corporate-proxy.com:8080

# Build arg null
$ docker build --build-arg HTTP_PROXY="" -f Dockerfile .
> 

# Build arg value
$ docker build --build-arg HTTP_PROXY="override" -f Dockerfile .
> override

Reproduce the reported Dockerfile example with the three HTTP_PROXY cases and inspect the Dockerfile processing and syntax-highlighting entry points that report dockerfile-utils(44). Confirm that the '-' modifier accepts the URL containing '/', then verify that the diagnostic disappears and the substitution is highlighted correctly.