title: `docker build -t` option accepted but no longer tags the new image
body: <p>I almost always build new versions of my Docker image by pasting this into the shell:</p>
  <pre><code>docker build -f Dockerfile  \
    -t ofiscal/tax.co:new .
  </code></pre>
  <p>(Once I see that it works I change the tag -- that is, the part after the colon.)</p>
  <p>Today when I tried that, the build succeeded, and yet when I run <code>docker images</code> I see &quot;&quot; listed for the new image under REPOSITORY and TAG.</p>
  <p>I was thereafter able to run <code>docker tag</code> to give it the name that I wanted, so I guess it's not a big deal, but I'd like to know what went wrong.</p>
  <p><code>docker --version</code> says I'm at <code>Docker version 20.10.9, build v20.10.9</code>. (That may have been updated from an older version on my system recently. I don't know how to find out.)</p>