No way to only keep tagged versions with min-versions-to-keep

[ramblingenzyme]

We were trying to use this action to clean up a Docker package/repo we use exclusively for build cache. This is because our actual images to go AWS ECR, which does not have production ready build cache support. aws/containers-roadmap#876

We also chose this over using GH Actions cache, as we haven't optimised our container size and filled the Actions cache quota very quickly.

We were trying to run this action twice in a workflow to one to delete all the untagged versions and one to clear out old tagged versions.

Because we've let our build cache images pile up, not all untagged images were deleted, and the second action, which uses min-versions-to-keep removed some tagged images. This was unexpected for us, as we thought that the action would ignore untagged images when considering which images to keep.

Essentially this change to src/delete.ts is what we expected.

However, I understand changing this behaviour would be a breaking change, and add further complexity to this action. It could instead be part of another input, but the inputs are very complex already. #110

We've also realised that this action is better used as an ongoing cleanup, rather than trying to make it go back and clean up what's already piled up. Perhaps some mention of this in the documentation could be helpful.