title: docker build error (apt-utils is not installed, cannot remove '/var/lib/apt/lists': Directory not empty)
body: <p>I'm trying to build a docker image using the following command</p>
  
  <pre><code>docker build -t blog .
  </code></pre>
  
  <p>and the Dockerfile:</p>
  
  <pre><code>FROM ruby:2.4
  
  
  RUN apt-get update -yqq \
      &amp;&amp; apt-get install -yqq --no-install-recommends \
        postgresql-client \
        &amp;&amp; rm -rf /var/lib/apt/lists
  
  
  WORKDIR /usr/src/app
  COPY Gemfile* .
  RUN bundle install
  COPY . .
  
  EXPOSE 3000
  CMD rails server -b 0.0.0.0
  </code></pre>
  
  <p>the result I get is the following:</p>
  
  <pre><code>Step 1 : FROM ruby:2.4
   ---&gt; ef296fcb8c7c
  Step 2 : RUN apt-get update -yqq     &amp;&amp; apt-get install -yqq --no-install-recommends       postgresql-client       &amp;&amp; rm -rf /var/lib/apt/lists
   ---&gt; Running in 18bfd303925d
  debconf: delaying package configuration, since apt-utils is not installed
  Selecting previously unselected package postgresql-client-common.
  (Reading database ... 21168 files and directories currently installed.)
  Preparing to unpack .../postgresql-client-common_165+deb8u2_all.deb ...
  Unpacking postgresql-client-common (165+deb8u2) ...
  Selecting previously unselected package postgresql-client-9.4.
  Preparing to unpack .../postgresql-client-9.4_9.4.10-0+deb8u1_amd64.deb ...
  Unpacking postgresql-client-9.4 (9.4.10-0+deb8u1) ...
  Selecting previously unselected package postgresql-client.
  Preparing to unpack .../postgresql-client_9.4+165+deb8u2_all.deb ...
  Unpacking postgresql-client (9.4+165+deb8u2) ...
  Setting up postgresql-client-common (165+deb8u2) ...
  Setting up postgresql-client-9.4 (9.4.10-0+deb8u1) ...
  update-alternatives: using /usr/share/postgresql/9.4/man/man1/psql.1.gz to provide /usr/share/man/man1/psql.1.gz (psql.1.gz) in auto mode
  Setting up postgresql-client (9.4+165+deb8u2) ...
  rm: cannot remove '/var/lib/apt/lists': Directory not empty
  The command '/bin/sh -c apt-get update -yqq     &amp;&amp; apt-get install -yqq --no-install-recommends       postgresql-client       &amp;&amp; rm -rf /var/lib/apt/lists' returned a non-zero code: 1
  </code></pre>
  
  <p>what is red are the 2 lines:</p>
  
  <pre><code>debconf: delaying package configuration, since apt-utils is not installed
  
  rm: cannot remove '/var/lib/apt/lists': Directory not empty
  </code></pre>
  
  <p>Why it cannot remove the directory since I'm using -rf?</p>
  
  <p>Thank you!</p>