Upgrade Dockerfile language server to 0.8.0

[rcjsuen]

This new release improves the semantic highlighting support for variables in addition to correcting a false positive linting error about empty continuation lines on here-documents. Please test these two new changes with the Dockerfile below.

FROM node:alpine
# the variables should now
# have symbols in a
# different color
RUN echo ${variable:+word}
RUN echo ${variable:-word}
# there should not be an
# empty continuation line
# error here
RUN <<EOT
    abc

    def
EOT

Screenshot taken with the "Tomorrow Night Blue" theme:

[rcjsuen]

Same as last month (#3355 (comment)), GitHub Actions doesn't want to run the build... 🤔

[bwateratmsft]

Awesome work as always @rcjsuen, thank you!