도커 우분투 apt-get update 에러 뜰때

도커로 우분투 설치 후 apt-get update가 안되는 상황

도커 공부를 좀 해볼려고 도커 공식 우분투 이미지를 다운로드 받아서 설치했는데, 리눅스를 깔고나면 가장 먼저 하게 되는.. sudo apt-get update를 하고 기분좋게 쳐다보려고 하는데 뭔 에러가 아래 처럼 주루룩 뜹니다.

Err:1 http://security.ubuntu.com/ubuntu bionic-security InRelease                                         
  Temporary failure resolving 'security.ubuntu.com'                                                       
Err:2 http://archive.ubuntu.com/ubuntu bionic InRelease                                                   
  Temporary failure resolving 'archive.ubuntu.com'                                                        
Err:3 http://archive.ubuntu.com/ubuntu bionic-updates InRelease                                           
  Temporary failure resolving 'archive.ubuntu.com'                                                        
Err:4 http://archive.ubuntu.com/ubuntu bionic-backports InRelease                                         
  Temporary failure resolving 'archive.ubuntu.com'                                                        
Reading package lists... Done                                                                             
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/bionic/InRelease  Temporary failure resolving 'a
rchive.ubuntu.com'                                                                                        
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/bionic-updates/InRelease  Temporary failure reso
lving 'archive.ubuntu.com'                                                                                
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/bionic-backports/InRelease  Temporary failure re
solving 'archive.ubuntu.com'                                                                              
W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/bionic-security/InRelease  Temporary failure re
solving 'security.ubuntu.com'                                                                             
W: Some index files failed to download. They have been ignored, or old ones used instead.

내가 뭘 잘못했는데....

찾아보니 DNS 문제거나 그걸로 해결이 안된다면 혹은 캐싱 문제로 우분투 도커 이미지를 삭제하면 된다고도 하는데 아래 답변들을 참고합시다.

• https://askubuntu.com/questions/881843/dns-issue-with-docker-image
• https://stackoverflow.com/questions/37706635/in-docker-apt-get-install-fails-with-failed-to-fetch-http-archive-ubuntu-com