Add a basic Devcontainer

[blue42u]

Development containers ("Devcontainers") are a way to describe a reproducible and automatically generated development environment, based off a Docker or Podman container. These allow new (and current) developers and contributors start writing and testing code faster and reduce the barrier for entry.

Devcontainers is still largely a VSCode feature, but is slowly gaining traction among IDEs (IntelliJ, Zed) and cloud-hosted development (GitHub Codespaces, Ona). In addition, Devcontainers is the development model supported by "atomic" Linux distros such Project Bluefin (part of Universal Blue).

This PR adds a boilerplate devcontainer spec based on the spack/ubuntu-noble:develop container image. The container is configured with a persistent volume for Spack installs, staging area in the container's /var/tmp (to avoid size issues if /tmp is a tmpfs), and builds for the generic architecture by default.

[wdconinc]

I'd review this in codespace but apparently I can't "for my own protection" since it's on a fork. Have you checked that codespaces handles this well?

[blue42u]

Ah, security.

I hadn't checked Codespaces specifically until just now, it worked fine except spack wasn't sourced properly because Codespaces doesn't use a login shell for the integrated terminal. It works fine on local VSCode, just not in Codespaces. Weird.

I've thrown in a quick patch to source setup-env.sh in non-login shells. With that, Codespaces seems fully functional:

[wdconinc]

Thanks for verifying this works in codespaces.