Handle duplicate DiffId docker image layers

[cobya]

When Docker layers are empty there is a possibility that the digest ID is constant between layer indexes. Previously that would cause an exception in the .ToDictionary call in LinuxScanner.cs where it expects unique keys.

[github-actions]

👋 Hi! It looks like you modified some files in the Detectors folder.
You may need to bump the detector versions if any of the following scenarios apply:

• The detector detects more or fewer components than before
• The detector generates different parent/child graph relationships than before
• The detector generates different devDependencies values than before

If none of the above scenarios apply, feel free to ignore this comment 🙂