title: GPG error in Ubuntu 21.04 after second apt-get update during Docker build
body: <p>Getting error while building the following Docker file</p>
  <pre><code>FROM ubuntu:21.04
  
  RUN apt-get update &amp;&amp; \
      apt-get install --no-install-recommends -y curl=7.\* &amp;&amp; \
      apt-get install --no-install-recommends -y unzip=6.\* &amp;&amp;\ 
      rm -rf /var/lib/apt/lists/*
  
  RUN apt-get update &amp;&amp; \
      mkdir -p /usr/share/man/man1 &amp;&amp; \
      apt-get install --no-install-recommends -y maven=3.6.3-5 &amp;&amp; \
      apt-get clean &amp;&amp; \
      rm -rf /var/lib/apt/lists/*
  </code></pre>
  <p>The error occurs when the second <code>apt-get update</code> runs.</p>
  <p>The error is as follows :-</p>
  <pre><code>E: The repository 'http://security.ubuntu.com/ubuntu hirsute-security InRelease' is not signed.
  W: GPG error: http://archive.ubuntu.com/ubuntu hirsute InRelease: gpgv, gpgv2 or gpgv1 required for verification, but neither seems installed
  E: The repository 'http://archive.ubuntu.com/ubuntu hirsute InRelease' is not signed.
  W: GPG error: http://archive.ubuntu.com/ubuntu hirsute-updates InRelease: gpgv, gpgv2 or gpgv1 required for verification, but neither seems installed
  E: The repository 'http://archive.ubuntu.com/ubuntu hirsute-updates InRelease' is not signed.
  W: GPG error: http://archive.ubuntu.com/ubuntu hirsute-backports InRelease: gpgv, gpgv2 or gpgv1 required for verification, but neither seems installed
  E: The repository 'http://archive.ubuntu.com/ubuntu hirsute-backports InRelease' is not signed.
  
  </code></pre>
  <p>Any kind of help would be appreciated.</p>