Update build.sh

fix the bug："cp: target './nginx/dist' is not a directory"

docker/build.sh

@@ -9,6 +9,7 @@ cd wvp-GB28181-pro/web_src && \
     npm run build 
     
 cd ../../
+mkdir -p ./nginx/dist
 cp -r wvp-GB28181-pro/src/main/resources/static/* ./nginx/dist
 
 echo "构建ZLM容器"
@@ -41,4 +42,4 @@ chmod +x ./build.sh
 ./build.sh
 cd ../
 
-./push.sh
+./push.sh