Fix crash when FORCE_PUSH is not set

Author: Karlinator

entrypoint.sh

@@ -31,7 +31,7 @@ sh -c "git config --global core.askPass /cred-helper.sh"
 sh -c "git config --global credential.helper cache"
 sh -c "git remote add mirror $*"
 sh -c "echo pushing to $branch branch at $(git remote get-url --push mirror)"
-if [ "$FORCE_PUSH" = "true" ]
+if [ "${FORCE_PUSH:-}" = "true" ]
 then
   sh -c "git push --force mirror $branch"
 else