Custom hash generation

[SlavenIvanov]

Hello!
For our current project we have a setup Prisma in our lambdas. This means that any change to the prisma folder would merit a lambda layer regeneration. From the docs I came to understand that only changes to a package.json trigger a new layer build.
I also saw that there is a customHash property, which could be used to watch for any potential changes.
Do you have any pointers to how I could watch for changes in both package.json and the prisma folder?