Pin Docker image to a specific SHA #201

Docker allows specifying a digest when pulling images using the <name>:<version>@sha256:<digest> syntax. This allows guarantees reproducibility and helps prevent supply chain attacks.

When using this action, it is currently possible to pin the Docker image to a specific version using the azcliversion input, but because of the extra validation it is not possible to suffix version numbers with a SHA256 digest.

The action should detect if such a prefix is present and strip it before checking the validity of the version number (but still use it when pulling the image).