Remediations should be disabled if docker file path is not correct

What does this MR do?

• Makes sure that disabled? function returns the right response when the docker_file path refers to a directory and not to a file. Before this change disabled? would be false if we provided a valid directory.
• Adds a test for this corner case

What are the relevant issue numbers?

Container Scanning - Do not attempt remediation... (gitlab-org/gitlab#378003 - closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer

/label typebug