Feature request: Allow for replacing FROM image in locked down environments

[viper-cool79]

Why?

This action is utilizing (like most I assume) a Dockerfile where the FROM points to ghcr.io. This might be blocked in certain circumstances and thus I wish for a way to input a custom image URL in a fashion similar to other settings in this action:

[...]
      - name: Run Trivy vulnerability scanner
        uses: aquasecurity/trivy-action@master
        with:
          base-image: 'sub.domain.tld/namespace/trivy:0.50.1'
[...]

This issue comes with a suggested and ugly drafted pull request, so if someone has a better idea on how to solve this without cloning the repo and switching this one FROM on the inside please feel free to suggest solutions.