Hello @Sgt. Red Pepper,
403 error is returned when the yarn or npm cli doesn't get the GSAP token value.
Usually yarn cli reads the .npmrc file in the project, but some versions require a .yarnrc file. It would be helpful to know which version of yarn you are using.
Also, can you make sure whether the env variable is added to the container/vpc? Please note that the variable needs to be exposed to the machine where the `yarn install` command is run. (Many projects don't expose env variables to docker instances hence 403 errors)
Let me know if this helps