title: Android's repo tool fails with python2
body: <p>My Dockerfile has <code>python3</code>, and I get:</p>
  <pre><code>RUN repo init -u https://android.googlesource.com/platform/manifest
   ---&gt; Running in f3adc38c2aac
  /usr/bin/env: 'python': No such file or directory
  </code></pre>
  <p>If I install <code>python</code>, I get this error:</p>
  <pre><code>Step 6/7 : RUN repo init -u https://android.googlesource.com/platform/manifest
   ---&gt; Running in 9c03e1b95c0f
  repo: warning: Python 2 is no longer supported; Please upgrade to Python 3.6+.
  Traceback (most recent call last):
    File &quot;/home/project/aosp/.repo/repo/main.py&quot;, line 56, in &lt;module&gt;
      from subcmds.version import Version
    File &quot;/home/project/aosp/.repo/repo/subcmds/__init__.py&quot;, line 38, in &lt;module&gt;
      ['%s' % name])
    File &quot;/home/project/aosp/.repo/repo/subcmds/upload.py&quot;, line 27, in &lt;module&gt;
      from hooks import RepoHook
    File &quot;/home/project/aosp/.repo/repo/hooks.py&quot;, line 472
      file=sys.stderr)
          ^
  SyntaxError: invalid syntax
  </code></pre>
  <p>Repo is downloaded like this:</p>
  <pre><code>RUN mkdir -p /android_tools \
  &amp;&amp; curl https://storage.googleapis.com/git-repo-downloads/repo &gt; /android_tools/repo \
  &amp;&amp; chmod a+x /android_tools/repo
  </code></pre>