Docker: What should I do when getting "503 service unavailable" error after executing docker pull command?

Getting "503 service unavailable" error after executing "docker pull" command is a very rare situation but there was in the past. So, it’s worth knowing for using docker. Most of the time, this is Docker Hub’ server side problem not client one. Once the error happens, we should visit their web site called Docker system status real-time view before struggling. When the error happened, may be "Docker Hub Registry" is in some trouble status.