I am getting following error without this commit when doing docker build.

  File "<string>", line 1, in <module>
  File "/usr/local/lib/python2.7/dist-packages/setuptools/__init__.py", line 12, in <module>
    import setuptools.version
  File "/usr/local/lib/python2.7/dist-packages/setuptools/version.py", line 1, in <module>
    import pkg_resources
  File "/usr/local/lib/python2.7/dist-packages/pkg_resources/__init__.py", line 70, in <module>
    import packaging.version
ImportError: No module named packaging.version

No idea why this change fix the problem, but it does fix.

- Put the 'ARG DEBIAN_FRONTEND=noninteractive' ahead of the first
  'apt-get install' invoke.
- Add comments to Dockerfiles to explain why we're adding Bazel
  installation.
Signed-off-by: NLi Peng <lipeng@unisound.com>

Also add DEBIAN_FRONTEND=noninteractive to suppress the debconf
messages.