* feat: support docker image history scanning
* refactor: collapse error handling into return
Style suggestion from review feedback.
* fix: associate layers with history entries
Where possible, add the associated layer to the history entry record. This may help tracing any issues discovered.
This also changes the entry reference format to `image-metadata:history:%d:created-by` which _may_ be more self-explanatory.
The previous implementation used int64 for both, which can be mixed up
easily. Using distinct types adds a layer of type safety checked by the
compiler.
* added functionality to scan docker images with digests instead of tags
* cleaned import statement
* added unit test for baseAndTag parsing + remote digest scan
* Implement CommonSourceUnitUnmarshaller
* Add SourceUnitUnmarshaller to all sources using
All sources, with the exception of git, will use the CommonSourceUnit as
they only contain a single type of unit to scan.
* Fix method comments to adhere to Go's style guide
