Detect lowercase prefixed Dockerfile names

[Mirochill]

Fixes #37

Summary:

• make Dockerfile candidate detection case-insensitive so prefixed lowercase names such as container-inspector.dockerfile are scanned
• parse Dockerfiles through a file object so custom Dockerfile filenames are handled by dockerfile-parse
• add a regression fixture and test for a lowercase prefixed Dockerfile name

Validation:

• Not run locally
• git diff --check HEAD~1..HEAD
• git show --check --stat --oneline HEAD
• Remote CI: Azure aboutcode-org.container-inspector build 18870 passed; DCO passed