fix Dockerfile parse error line

#2022

workspace/Dockerfile

@@ -141,7 +141,7 @@ ENV COMPOSER_AUTH ${COMPOSER_AUTH}
 
 RUN if [ ${COMPOSER_AUTH} = false ]; then \
     # remove the file
-    rm /home/laradock/.composer/auth.json
+    rm /home/laradock/.composer/auth.json \
 ;fi
 
 ARG COMPOSER_REPO_PACKAGIST