What parts go under source code version control?
There are different ways to approach this, depending on what you need. The easiest approach is the one you described in #1 (Put the entire Drupal folder under version control).

That allows you to easily checkout the full working copy of the codebase with just Git. The downside is that you are storing a lot of code in your Git repo which already exists in other Git repos (ie: Drupal core, contrib modules, etc).

With Drupal 8, the community is generally moving towards using Composer for managing dependencies and building the Drupal codebase. Then, you only need a composer.json file (which points to specific versions of Drupal core and contrib projects), and your custom code in the Git repository. More information: https://www.drupal.org/docs/develop/using-composer/using-composer-with-drupal

Another option is to create an installation profile, and use Drush Make. This method is deprecated in favor of Composer in Drupal 8, but still works nicely for Drupal 7 projects. This is better suited to common code/configuration that you might use to set up lots of similar sites, rather than one highly-customized and specific site.

Regarding Docker - that's sort of a separate decision. You'll still need to choose one of the above options, and then build the codebase accordingly inside the Docker container (or mount it in a volume).