Remove usage of dockerfile dependency in pipeline

Background

Currently, we're using the dockerfile package to parse project Dockerfiles and evaluate whether the final FROM in a dockerfile matches the format we expect. We should update this code to no longer depend on this package so we can remove it as a dependency from the pipeline image.

Acceptance Criteria

• file_parser.py is updated to no longer use the dockerfile package
• The dockerfile package has been removed from the image
• The updated image and script have been tested in staging

Definition of Done

• Write or update any unit or integration tests
• Project pipeline runs successfully
• Add any applicable checkboxes for testing e.g. Ran pipeline in staging env or pipeline-test-project pipeline run
• Solution is captured as code and/or documentation and merge requests have been submitted
• Code review completed and merge request approved/merged
• All Acceptance Criteria have been completed