Add DevContainer (GitHub codespaces support and convenience commands)

[SeanKilleen]

Makes it easier for folks (and maintainers!) to drop into a web-based or local vscode instance and have everything you need to build & run the site locally.

This container pulls from https://github.com/SeanKilleen/blog-in-a-box-container/ , which I maintain.

In addition to a devcontainer, it adds several shortcuts for ease of use:

• localdev - serves with livereload
• build and serve
• spellcheck - runs cSpell (once my follow-up PR is complete to set up the config)
• lint - runs markdown linting (once my follow-up PR is complete to set up the config)
• lintfix runs markdown linting with fixes. (once my follow-up PR is complete to set up the config)

[SeanKilleen]

Marking this as a draft so I can ensure it works locally in my vscode for this repo.

[SeanKilleen]

It was time for an update to the devcontainer to support GH pages v232, so diverting to work on that a little.

[SeanKilleen]

This is ready! You can verify by opening a GitHub codespace on this branch right in your browser (or opening this branch in local vscode and then choosing "rebuild and reopen in container" from the command palette. That will load the devcontainer that's in this branch. Once it loads, you can type build or serve from the terminal. If you run serve, you'll be able to see it start up right within github codespaces.

Just verified it myself 👍 Happy to walk through it with you if you'd like.

[SeanKilleen]

@merill this one is ready for review 👍 (won't bug you any more on it)

[merill]

This is amazing. I've never used codespaces and this is a god send compared to fighting rails versions...

Thanks you so much @SeanKilleen !