Test or cull Docker support

### Versions

* Buildozer: 2.2.0

### Description

I have rumours from several sources to avoid Buildozer's Dockerfile.

The strongest version is from the (unofficial) [Android-For-Python-Users](https://github.com/Android-for-Python/Android-for-Python-Users/):

> There is an **obsolete** Docker image. **DO NOT USE IT.** The Docker image is not maintained.

However, it has every appearance of being supported:

* It is [occasionally edited](https://github.com/kivy/buildozer/commits/master/Dockerfile).
* It is [documented](https://github.com/kivy/buildozer#readme).
* It is [very, very lightly tested](https://github.com/kivy/buildozer/blob/master/.github/workflows/test_python.yml).
- The test is simply to run `buildozer --version`.

If the rumour is true, and the Dockerfile is **not** being maintained:
* Remove the Dockerfile.
* Remove the vestigial test.
* Remove mentions from the documentation.
* Put a PR against the unofficial guide to update that advice.

I am happy to help if this is the direction wanted.

If the rumour is false, and the Dockerfile is being maintained, then let's demonstrate it clearly by running the test suite on Docker.

* Modify the `test_python.yml` (I can do this with some assistance, or someone else more au fait with docker commands can do it)
* Put a PR against the unofficial guide to update that advice.

Review the repository Dockerfile, .github/workflows/test_python.yml, and the Docker references in the README, then compare them with the concerns in the linked unofficial guide. Determine whether Docker support is maintained enough to test or whether the Dockerfile, vestigial test, and documentation references should be removed; done requires a clear project decision and corresponding updates.

Written by the indexing model from the issue text.