fix: Get the right input variable

see 
https://docs.github.com/en/actions/creating-actions/creating-a-docker-container-action#writing-the-action-code

Author: lcenchew

entrypoint.sh

@@ -3,6 +3,5 @@
 cd "${GITHUB_WORKSPACE}"
 
 git remote add -f upstream "https://github.com/$1.git"
-git branch ${GITHUB_REF}
-git merge --ff-only upstream/${GITHUB_REF}
+git merge --ff-only upstream/$2
 git push