Allow for easy testing with Docker

[alxy]

This adds a github action workflow to push a ready-to-test Docker image including a functional October isnstallation along with the Test plugin installed to GitHub Container registry.

For this to work, the following (beta) feature will need to be enabled: Improved container support

Demo of the workflow on a PR to my fork.

The PR also adds a docker-compose file to easily test locally. Workflow would be the following:

$ git clone https://github.com/octoberrain/test-plugin.git
$ cd test-plugin
$ docker-compose up -d
Recreating oc-test-plugin_web_1 ... done

Then visit http://localhost/backend and login via admin/admin.