Set up devcontainer/Codespaces configuration for this repo

[lbussell]

With #1393, this repo will likely adopt templates. Adopting templating in this repo means that even simple changes to Dockerfiles will require running Docker to generate the templated content (that's how the ImageBuilder-based infrastructure works).

To reduce friction for making simple changes to the repo, we should do the following:

• Add devcontainer configuration similar to https://github.com/dotnet/dotnet-docker/blob/a93e9cc9fae5540a2108ab30cd4c83cf0132069f/.devcontainer/devcontainer.json
• Add common tasks for generating templated content similar to https://github.com/dotnet/dotnet-docker/blob/a93e9cc9fae5540a2108ab30cd4c83cf0132069f/.vscode/tasks.json#L5-L56
• Ensure the devcontainer is pre-built on a schedule so that it takes less time to get started with changes