title: why does docker prune not remove my dangling images?
body: <p>Tthe docs say docker prune "removes all dangling images". And it even issues a warning "this will remove all dangling images". So why does it not remove mine?</p>
  
  <pre><code> ubuntu@ip-172-31-26-48:~$ docker image prune
  WARNING! This will remove all dangling images.
  Are you sure you want to continue? [y/N] y
  Total reclaimed space: 0 B
  ubuntu@ip-172-31-26-48:~$ docker images
  REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
  simonm3/registrm    latest              bd3ef6d73785        7 minutes ago       468 MB
  simonm3/registr     latest              7dd15943671b        2 hours ago         2.51 GB
  simonm3/registrm    &lt;none&gt;              f8f22986e85a        45 hours ago        2.93 GB
  simonm3/registrm    &lt;none&gt;              c1b9d8606371        45 hours ago        2.93 GB
  simonm3/registrm    &lt;none&gt;              f1ca97467106        45 hours ago        2.93 GB
  simonm3/registr     &lt;none&gt;              6a5af88f93a7        45 hours ago        4.42 GB
  simonm3/registrm    &lt;none&gt;              93dc81b0fa7a        45 hours ago        2.93 GB
  simonm3/registrm    &lt;none&gt;              dbc50510d499        45 hours ago        2.93 GB
  simonm3/registr     &lt;none&gt;              0101ca5d4a25        45 hours ago        4.42 GB
  simonm3/registrm    &lt;none&gt;              d8f85df39f0a        46 hours ago        2.93 GB
  simonm3/registrm    &lt;none&gt;              053892e9798b        46 hours ago        2.93 GB
  simonm3/registrm    &lt;none&gt;              3845056cf241        46 hours ago        2.92 GB
  simonm3/registr     &lt;none&gt;              349ef3c589bd        46 hours ago        4.42 GB
  simonm3/registrm    &lt;none&gt;              5dcbb0db4710        2 days ago          2.92 GB
  simonm3/registr     &lt;none&gt;              e403f74861f6        2 days ago          4.32 GB
  simonm3/registr     &lt;none&gt;              c12ca8819c24        5 days ago          4.32 GB
  simonm3/registrm    &lt;none&gt;              4bb8ed8154b7        6 days ago          2.92 GB
  ubuntu@ip-172-31-26-48:~$ docker images -q -f dangling=true
  f8f22986e85a
  c1b9d8606371
  f1ca97467106
  6a5af88f93a7
  93dc81b0fa7a
  dbc50510d499
  0101ca5d4a25
  d8f85df39f0a
  053892e9798b
  3845056cf241
  349ef3c589bd
  5dcbb0db4710
  e403f74861f6
  c12ca8819c24
  4bb8ed8154b7
  </code></pre>