pin dependency image to SHA256 digest

[listx]

Docker-based GitHub actions are built every time [1]. This means that if the nnichols/clojure-dependency-update-action:latest image is compromised, it could result in arbitrary code execution. Pinning the image to its current SHA256 digest [2] removes that attack vector.

[1] https://docs.github.com/en/actions/creating-actions/about-custom-actions#docker-container-actions [2] https://hub.docker.com/layers/nnichols/clojure-dependency-update-action/latest/images/sha256-06c47e969b386796a09f296d80af705c1d8b578cae41ebe018b08a0f657d4081?context=explore

Proposed Changes

• Additions:
• Updates: Dockerfile FROM usage
• Deletions:

Pre-merge Checklist

• [x] Write + run tests
• [x] Update CHANGELOG and increment version
• [x] Update README and relevant documentation

[nnichols]

Thanks for the contrib!