Is your feature request related to a problem? Please describe.
When developers are building wasm wrappers with the vm build strategy, the only way to customize it is to modify the build script located at .polywrap/wasm/build/project/polywrap-build.sh directory, and run the docker command manually.

Describe the solution you'd like
Developers should be able to set the build script within the build manifest, something similar to this:

strategies:
  vm:
    build_script: ./custom.sh