title: pipenv sync and pipenv install --system --ignore-pipfile in docker environment
body: <p>According to <a href="https://pipenv.readthedocs.io/en/latest/" rel="noreferrer">pipenv official documentation</a>:</p>
  <blockquote>
  <p><strong>sync</strong></p>
  <p>pipenv sync [OPTIONS]</p>
  <p>Installs all packages specified in Pipfile.lock.</p>
  <p><strong>install</strong></p>
  <p>pipenv install [OPTIONS] [PACKAGES]...</p>
  <p>Installs provided packages and adds them to Pipfile, or (if no packages are given), installs all packages from Pipfile.</p>
  <p>--ignore-pipfile
  Ignore Pipfile when installing, using the Pipfile.lock.</p>
  </blockquote>
  <p>Is it safe to assume <code>pipenv sync</code> and <code>pipenv install --ignore-pipfile</code> are identical without any hidden drawbacks?</p>
  <p><em><strong>More background</strong>: I was using --system flag to install the python packages to the system since I don't care about isolated environments in a docker container. However --system flag is unavailable for <code>pipenv sync</code> (See <a href="https://github.com/pypa/pipenv/issues/2227" rel="noreferrer">github issue</a>), so I figured <code>pipenv install --system --ignore-pipfile</code> might be a viable hack.</em></p>