Remove Dockerfile and VARIANT argument, update README, start using

[johnbullnz]

Purpose

• Fix indentation in devcontainer.json
• Update repository as per Dockerfile and VARIANT argument are no longer required #4
• Start using devcontainers org images as per the note on https://github.com/microsoft/vscode-dev-containers/tree/main/containers/python-3.

Does this introduce a breaking change?

[ ] Yes
[x] No

Pull Request Type

What kind of change does this Pull Request introduce?

[ ] Bugfix
[ ] Feature
[x] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[x] Documentation content changes
[ ] Other... Please describe:

How to Test

• Rebuild the devcontainer

What to Check

Verify that the following are valid

• Dockerfile is indeed no longer required and the preferred image is the devcontainer org image.
• Updates to README text.

Other Information

N/A

[pamelafox]

Awesome, thanks so much!