Yarn Package invalid key

If your environment has failed due to an issue with the yarn package, similar to below, it means there's an issue with the package's GPG signature. 

GPG error: https://dl.yarnpkg.com/debian stable InRelease: The following signatures 
were invalid: EXPKEYSIG 23E7166788B63E1E Yarn Packaging <yarn@dan.cx>

This can be resolved by adding following to the top of your environment's Dockerfile to refresh the package key:  

RUN curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | sudo apt-key add - 

If you're seeing a similar error but for an nvidia key, please check out this article: Build error for nvidia GPG key