fix: set docker-bench-security to sh

[denhamparry]

• Closes bash not available within Alpine:3.13  #473
• Set script to execute with sh rather than bash
• Dockerfile is based on Alpine 13.3 that has sh and not bash

[konstruktoid]

Thanks for this PR @denhamparry, I forgot to fix the Alpine Dockerfile when changing to bash.
My suggested change is instead:
ENTRYPOINT [ "/usr/bin/dumb-init", "/bin/sh", "docker-bench-security.sh" ]

[denhamparry]

Thanks for the review @konstruktoid 😄 I agree your suggestion is better and have tested it locally and it fixes the issue.

Updated the PR accordingly 👍

[konstruktoid]

Thanks for the update @denhamparry