Document act usage to run GitHub action locally

[ghost]

closes #431

Summary:

This PR updates the documentation to describe how to run the end to end workflow locally in Docker via the act project
https://github.com/nektos/act

Expected behavior:

Syncing the latest version of master and passing the command act -j run-on-data
Now works fine with the validator running on all datasets until the persist artifact steps...

.. which is a known issue of act
nektos/act#329

The files in question can be manually collected inside the container.
Datasets .zip files are present as well as validation reports .json in the output folder

Please make sure these boxes are checked before submitting your pull request - thanks!

• Run the unit tests with gradle test to make sure you didn't break anything
• Format the title like "Fix #<issue_number> - " (for example - "Fix Bug: Running using documented Docker documentation fails #1111 - Check for null value before using field")
• Linked all relevant issues
• Include screenshot(s) showing how this pull request works and fixes the issue(s)

[lionel-nj]

LGTM! A few comments in-line though.