fix: exclude irrelevant directories from asset hashing

[eoinsha]

Existing assetExcludesoptions only applies to the rsync step in copying files to the docker container for building. The asset hashing, used to detect source changes, was still being applied to everything in the root directory.
This changes provides defaults (configurable) to exclude cdk, .venv, etc.

[chrismcg]

I have a memory of trying something like this back in November and it not working for nested folders. I think it might have been before #5 though.

I assume you've seen improvements with this in your real world use cases?

[eoinsha]

Yes @chrismcg - tested and seems to work well