chore(gha): use git describe for git-rev.txt helper

.github/workflows/dockerbuild.yml

@@ -52,7 +52,7 @@ jobs:
           if [[ $GITHUB_REF == refs/tags/* ]]; then
             echo ${GITHUB_REF#refs/tags/} > git-rev.txt
           else
-            echo ${GITHUB_SHA::8} > git-rev.txt
+            git describe --exclude 'chart-*' > git-rev.txt
           fi
       -
         name: Set up Docker Context for Buildx