adding '\' to the Dockerfile nginx statement

nginx/Dockerfile

@@ -9,7 +9,7 @@ ADD nginx.conf /etc/nginx/
 ARG CHANGE_SOURCE=false
 RUN if [ ${CHANGE_SOURCE} = true ]; then \
     # Change application source from dl-cdn.alpinelinux.org to aliyun source
-    RUN sed -i 's/dl-cdn.alpinelinux.org/mirrors.aliyun.com/' /etc/apk/repositories
+    RUN sed -i 's/dl-cdn.alpinelinux.org/mirrors.aliyun.com/' /etc/apk/repositories \
 ;fi
 
 RUN apk update \