Refactor inputs to support docker/metadata-action

[ntkme]

This PR adds support for docker/metadata-action@v2. See #74.

Description

Support tags input in full image name form. When full image name form is used, input image would be optional.

Related Issue(s)

Closes #74

Checklist

• This PR includes a documentation change
• This PR does not need a documentation change

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• This PR includes test changes
• This PR's changes are already tested

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• This change is not user-facing
• This change is a patch change
• This change is a minor change
• This change is a major (breaking) change

Changes made

• Input image is now declared as required: false, and checked during runtime.
• If all tags are in full image name form, the tags are used as is.
• tags input are split by /\s+/ instead of " " because docker/metadata-action's output is newline separated.

[divyansh42]

Thanks for your contribution 👍
I think this PR is very accurate and perfect as per implementation.
The only thing that needs to be done is to update the input description (as we have changed those) in action.yml and in README.

[divyansh42]

Do you want to update/improve the documentation to clearly explain the changes? (In the same way, suggested at redhat-actions/push-to-registry#50 (comment))

[ntkme]

@tetchel Done with refactoring and added documentation. PTAL. Thanks!

[divyansh42]

Output image will be empty when using FQIN, can we add a note about this in the README output section?
Also, it will be great if we can add another example for FQIN in output tags and image-with-tag in README output section.

[tetchel]

lgtm, i'm going to hold this until the PTR one is approved too and then we can merge both and make sure they make sense together.