checkout release branch

.github/workflows/docker-build.yml

@@ -14,6 +14,7 @@ jobs:
         uses: actions/checkout@v2
         with:
           fetch-depth: 0
+          ref: ${{ github.event.release.target_commitish }}
       - name: Get the version
         id: get_version
         run: echo ::set-output name=VERSION::${GITHUB_REF#refs/tags/}