fix: strip v prefix from Docker image tags on release

GITHUB_REF_NAME includes the v prefix (e.g. v1.0.0-rc.4); Docker image
tags should not (e.g. 1.0.0-rc.4). Regression introduced in 78758f6.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: coopernetes

.github/workflows/docker-publish.yml

@@ -200,7 +200,7 @@ jobs:
 
       - name: Promote build image to release tags
         run: |
-          VERSION=${GITHUB_REF_NAME}
+          VERSION=${GITHUB_REF_NAME#v}
           SOURCE="${IMAGE}@${{ steps.build.outputs.digest }}"
 
           if [[ "${VERSION}" == *-* ]]; then