add devcontainer spec

[casperdcl]

• CPU-only default for gh codespaces
• GPU option (e.g. using https://containers.dev/implementors/templates/#folder-structure)?

Main problem: at least some devcontainer tools (e.g. VSCode extensions) and docker-stacks both fix shared folder user perms... in incompatible ways. Probably need to disable the clandestine magic of one of them. And/or open an issue at jupyter/docker-stacks

follow-up to #718