[CI] Nightly tag removed for release branch

[nwangfw]

Pull Request Description

Fixed the issue that previously tagged branches also get "nightly" tag

Related Issues

Resolves: #315

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[Jeffwan]

1. .github/workflows/docker-push-images.yml is for main branch image pushing, it should be always nightly. Let's update the release workflow.
2. there's no need to override the tag, we can simplicity use enable or not, like a boolean here.
3. by default night tag should be enabled, if not, let's skip nightly build & push

Alternatively, we can check git_commit_hash is ref_name or commit hash. if that can be done, we can choose that way as well

[nwangfw]

1. .github/workflows/docker-push-images.yml is for main branch image pushing, it should be always nightly. Let's update the release workflow.
2. there's no need to override the tag, we can simplicity use enable or not, like a boolean here.
3. by default night tag should be enabled, if not, let's skip nightly build & push

Alternatively, we can check git_commit_hash is ref_name or commit hash. if that can be done, we can choose that way as well

Modified based on your suggestions. Thanks!

[Jeffwan]

the change looks good to me