How to use App Platform's bindable environment variables in Dockerfile

I was following the tutorial on How to Use Environment Variables in App Platform, and I am unable to figure out how to use these convenience variables in my Dockerfile.

I am specifically trying to print the COMMIT_HASH to a file in the container of my web component.

ARG web.COMMIT_HASH=unspecified
RUN echo ${web.COMMIT_HASH} > /www/commithash.txt

I’ve also tried pulling it in using ENV but that fails during build.

ENV GITHASH=${web\.COMMIT_HASH:-unspecified}
RUN echo ${GITHASH} > /www/githash.txt

It errors out at the . dot.

So is there a way to get these convenience environment variables into my Dockerfile?

Any help is greatly appreciated.