tag `21` mentioned in the README but doesn't exist on Docker Hub or Github Packages

[KhalidAlansary]

$ docker pull docker.io/xianpengshen/clang-tools:21
Error response from daemon: manifest for xianpengshen/clang-tools:21 not found: manifest unknown: manifest unknown

[shenxianpeng]

Thank you for reporting the issue. I forgot to publish the images after PR #106 was merged.

Now the tag 21 has been successfully pushed to both Docker Hub and GitHub Packages.