Development container

[donkirkby]

Set up a minimal development container for the first item below. Other enhancements might be pulled to separate issues.

• [x] edit a markdown tutorial in the GitHub editor, and preview the changes before committing
• [x] edit Python code in the GitHub editor, and run the unit tests before committing
• [x] edit Python code in the GitHub editor, and see the changes in a browser tutorial before committing
• [ ] edit Python code or Java code in the GitHub editor, and download a plugin jar to test in PyCharm, before committing
• [ ] edit Python code or Java code in the GitHub editor or in a local docker container, and use IDEA to step through the code

JetBrains isn't yet listed on containers.dev, but their Gateway application sounds similar.

When we launch a Jekyll server, it sounds like GitHub will open a public port for it.

There might be some relevant details in Sarah Kaiser's slides, when they're published.