Switch to Colab's public image as the base.

[metrizable]

A first pass of using Colab's public image as the base image. This is a rough, minimalist/brute force, first attempt at making the switch. None of the libraries provided by Colab's are used. A few things of note:

• Installing miniconda was needed as Colab doesn't provide it. This likely can be avoided to just using micromamba with a few tweaks.
• A pip warning for BigQuery_Helper was resolved by passing in --use-pep517
• Some hackery with nltk was performed, which with a little effort, should be resolved. It was likely an artifact of an earlier iteration
• The entrypoint needed to be overridden, as Colab's image specifies a run script entrypoint, which clobbers testing

A few other rough edges and improvements should be made.

[djherbis]

Thanks again for the initial PR for this, now that we're Colab based I'll close this one :)