Docker: pre-build script does not actually run in docker build

[sharunkumar]

https://github.com/aeharding/voyager/actions/runs/11791484474/job/32843572978#step:7:607

possibly bash is not available in the base image

suggested fix: change it to sh "$SETUP_SCRIPT"

also the path needs to be updated: SETUP_SCRIPT="./scripts/disable_in_app_purchases.sh"

I've checked it locally, will raise a PR in a bit