Add GitHub Actions cache to Docker build

Use GHA cache backend to persist Docker layer cache between runs,
avoiding full renv package reinstalls when renv.lock is unchanged.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

Author: pcantalupo

.github/workflows/docker-publish.yml

@@ -29,6 +29,8 @@ jobs:
         with:
           context: .
           push: true
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
           tags: |
             virushunter/viewseurat:${{ steps.version.outputs.version }}
             virushunter/viewseurat:latest