Bring back the CI badge with the new URL scheme

Author: stepchowfun

README.md

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 # Docuum: LRU eviction of Docker images
 
+[![Build status](https://github.com/stepchowfun/docuum/actions/workflows/ci.yml/badge.svg?branch=main)](https://github.com/stepchowfun/docuum/actions?query=branch%3Amain)
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 *Docuum* performs least recently used (LRU) eviction of Docker images to keep the disk usage below a given threshold.
 
 Docker's built-in `docker image prune --all --filter until=…` command serves a similar purpose. However, the built-in solution isn't ideal since it uses the image creation time, rather than the last usage time, to determine which images to remove. That means it can delete frequently used images, which may be expensive to rebuild or time-consuming to pull.