Typo in `onbuild` environment variables?

[MoriTanosuke]

I tried to use the onbuild tag for my personal blog today and noticed that I had to set the environment variable HUGO_DESTINATION instead of HUGO_DESTINATION_ARG to switch the output folder to my volume.

Is this a typo in the README or did I miss some other configuration for the onbuild? 🤔

[klakegg]

I'm unable to find a typo. Please clarify where I may find the typo.