Adding a devcontainer.json and a button for easy codespacing

[tacastillo]

Why

Because it'll allow anyone with a GitHub account to have Dagster running and editable in roughly a minute.

Next Steps

If they have the dev container extension for VS Code/Jetbrains, it'll work OOB and ask the user if they want to open up the project in a container locally.

To get the snappiness of Codespaces though, we'd need to set up a pre-build on GitHub. It makes a pretty lean image and GitHub will automatically use it when turning on a Codespace.

I don't have admin permissions on this repo, so I can't set it myself. If someone is too lazy to figure out how to add me but also is energetic enough to try it themselves:

1. Head to https://github.com/dagster-io/biweekly-demo/settings/codespaces
2. Click the green Set up prebuild button
3. Have it set to run on every configuration change. It's technically not greedy enough, but with any changes we make, we can kick off a manual rebuild. (see pic below)
   

Hit save and it'll take about 5 minutes to get built. To monitor this, peep the GitHub Actions tab. After that, click on the Open in GitHub Codespaces button in the README, click Create Codespace, give it a minute to spin up, and run dagster dev once it's warmed up.

Wrinkle: idk how this all plays in with the Dagster GH org. I'm currently using this pattern on a repo on my GitHub account, so unsure how GH is gonna treat the GHA minutes or the storage for the image on this org.

[github-actions]

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[yuhan]

i don't have admin perm either but will ask