Local integration testing updates

[jrdnbradford]

I had a couple issues running integration tests locally recently and made a couple of updates and fixes that could be helpful:

1. The integration test docs did not include building the image needed to run the tests. This is now included.

2. The main test example

.github/integration-test.py run-test basic-tests \
  test_hub.py \
  test_proxy.py \
  test_install.py \
  test_extensions.py

would throw an error because no installer args are passed. installer_args being None would cause

' '.join(installer_args) 

to throw an exception.

I added an empty list as the default value to --installer-args to correct this and made a couple other updates to the docs/examples.

3. I made some major updates to the run_test help text so that usage can be generated with

.github/integration-test.py run-test --help

This is referenced in the docs.

🚀