title: apt-get update fails inside Dockerfile
body: <p>When I try to build a simple docker image:</p>
  <p>in Dockerfile:</p>
  <pre><code>FROM ubuntu:18.04
  
  RUN apt-get update
  </code></pre>
  <p>The output is:</p>
  <pre><code>$ sudo docker build --no-cache -t testbuild .
  Sending build context to Docker daemon  3.471MB
  Step 1/2 : FROM ubuntu:18.04
   ---&gt; c090eaba6b94
  Step 2/2 : RUN apt-get update
   ---&gt; Running in 53d64603442f
  Ign:1 http://archive.ubuntu.com/ubuntu bionic InRelease
  Ign:2 http://security.ubuntu.com/ubuntu bionic-security InRelease
  Ign:3 http://archive.ubuntu.com/ubuntu bionic-updates InRelease
  Err:4 http://security.ubuntu.com/ubuntu bionic-security Release
    404  Not Found [IP: 91.189.88.142 80]
  Ign:5 http://archive.ubuntu.com/ubuntu bionic-backports InRelease
  Err:6 http://archive.ubuntu.com/ubuntu bionic Release
    404  Not Found [IP: 91.189.88.152 80]
  Err:7 http://archive.ubuntu.com/ubuntu bionic-updates Release
    404  Not Found [IP: 91.189.88.152 80]
  Err:8 http://archive.ubuntu.com/ubuntu bionic-backports Release
    404  Not Found [IP: 91.189.88.152 80]
  Reading package lists...
  E: The repository 'http://security.ubuntu.com/ubuntu bionic-security Release' does not have a Release file.
  E: The repository 'http://archive.ubuntu.com/ubuntu bionic Release' does not have a Release file.
  E: The repository 'http://archive.ubuntu.com/ubuntu bionic-updates Release' does not have a Release file.
  E: The repository 'http://archive.ubuntu.com/ubuntu bionic-backports Release' does not have a Release file.
  The command '/bin/sh -c apt-get update' returned a non-zero code: 100
  </code></pre>
  <p>I tried:</p>
  <ol>
  <li><p>build with different types of ubuntu images - e.g. ubuntu:16.04, 18.04,20.04 - and get a similar error '...Release' does not have a Release file.'</p>
  </li>
  <li><p>I completely removed and reinstalled docker from my system, inc all containers and repository, and rebooted, and get the same error</p>
  </li>
  </ol>
  <p>I previously built multiple docker images and everything worked fine. And now I get this error out of nowhere all the time.</p>