title: Docker Could not resolve 'deb.debian.org'
body: <p>I was trying to build a container in Docker for some experiments. Here is my <code>Dockerfile</code>.</p>
  
  <pre><code>FROM debian
  
  RUN mkdir -p /var/run/sshd
  
  RUN apt-get update
  RUN apt-get install -y openssh-server
  
  RUN apt-get install -y sudo
  
  RUN echo AddressFamily inet &gt;&gt; /etc/ssh/sshd_config
  
  ARG username=Rivers
  ARG userpasswd=perfectXJ2017
  
  RUN useradd -ms /bin/bash $username &amp;&amp; (echo $username:$userpasswd | chpasswd)
  
  RUN adduser $username sudo
  
  CMD /usr/sbin/sshd -D
  </code></pre>
  
  <p>I tried to build my image with the command <code>sudo docker build -t ics-image .</code>.</p>
  
  <p>But then I got some error messages and the whole process stopped. Here is the error messages.</p>
  
  <pre><code>Sending build context to Docker daemon  2.048kB
  Step 1/11 : FROM debian
   ---&gt; 8626492fecd3
  Step 2/11 : RUN mkdir -p /var/run/sshd
   ---&gt; Running in 1e1f2dbbe5ca
  Removing intermediate container 1e1f2dbbe5ca
   ---&gt; dd4bec2f81d4
  Step 3/11 : RUN apt-get update
   ---&gt; Running in 022301215bfb
  Get:1 http://security.debian.org/debian-security stretch/updates InRelease [94.3 kB]
  Get:2 http://security.debian.org/debian-security stretch/updates/main amd64 Packages [450 kB]
  Err:3 http://deb.debian.org/debian stretch InRelease
    Could not resolve 'deb.debian.org'
  Err:4 http://deb.debian.org/debian stretch-updates InRelease
    Could not resolve 'deb.debian.org'
  Fetched 544 kB in 20s (26.9 kB/s)
  Reading package lists...
  W: Failed to fetch http://deb.debian.org/debian/dists/stretch/InRelease  Could not resolve 'deb.debian.org'
  W: Failed to fetch http://deb.debian.org/debian/dists/stretch-updates/InRelease  Could not resolve 'deb.debian.org'
  W: Some index files failed to download. They have been ignored, or old ones used instead.
  Removing intermediate container 022301215bfb
   ---&gt; 054d32710b62
  Step 4/11 : RUN apt-get install -y openssh-server
   ---&gt; Running in 802d2fa37b8d
  Reading package lists...
  Building dependency tree...
  Reading state information...
  Package openssh-server is not available, but is referred to by another package.
  This may mean that the package is missing, has been obsoleted, or
  is only available from another source
  
  E: Package 'openssh-server' has no installation candidate
  The command '/bin/sh -c apt-get install -y openssh-server' returned a non-zero code: 100
  </code></pre>
  
  <p>I did all this on Ubuntu 18.04. I cannot understand why this happened and how to solve this problem. Can anyone help?</p>