Support for buildpack volumes

In my humble opinion, a neat feature for this image would be the ability to specify volumes for the buildpack build. For example, this would be useful for paketo buildpack bindings.

I suggest defining a new variable BUILDPACK_VOLUMES and processing it in src/build.sh just like it already happens with BUILDPACK_URL and AUTO_DEVOPS_BUILD_IMAGE_FORWARDED_CI_VARIABLES.

Would this be a reasonable thing to implement here? Would you agree with the implementation suggestion?