Pin Docker image to a specific SHA

Docker allows specifying a digest when pulling images using the `:@sha256:` 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](https://github.com/Azure/cli/blob/9eb25b8360668fb0ecbafa808d40e2197b2f5f52/src/main.ts#L96) 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).