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

[gnowland]

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: docker/docs#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

[rcjsuen]

Thank you for your bug report. The parser is implemented on a best effort basis based on the documentation. Will see how things go with docker/docs#23165...

[rcjsuen]

Note that you can put # dockerfile-utils: ignore on the line before it to suppress the error.