- 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.

* But not reproduce the problem.

 when merge with the develop branch, some mistakes happened

* for modern language properties

When the define_py_data_sources2 has both train_list and test_list,
for job=test, the trainer will create both dataProvider_ and testDataProvider_.
But dataProvider_ is not used. This causes SIGSEGV at finishAsync() because asyncLoader_ is not created.

Change-Id: If579f715f80a70ebc795094792c3436bfa0f5746