What would you like to be added:
A less expensive deriveID function for file source

Why is this needed:
We discussed on the OSS community meeting on 22nd May that the file source deriveID function is a little peculiar in that it tries to SHA256 hash the entire contents of the file being scanned.

This gets expensive for users doing many file source scans.

We discussed on the call whether hashing the file contents is necessary, or whether we could avoid hashing file contents and just always hash the file (this is in fact the fallback if attempting to open the file errors). In addition there were discussions around using a more lightweight hash function to improve performance.

The Anchore team asked that I raise an issue here so that it could be discussed further.

Additional context: