Devcontainers should not set SHELL in the environment

[bk2204]

At work, we're using a devcontainer built from mcr.microsoft.com/vscode/devcontainers/universal:2-focal in GitHub Codespaces. Unfortunately, this image sets the SHELL environment variable to /bin/bash, which is not correct.

As an example of the problems this causes, I use zsh and have it set as the default login shell in my codespace. However, since SHELL is set to /bin/bash, when I open tmux, it spawns bash, not zsh.

POSIX states about this environment variable that “[t]his variable shall represent a pathname of the user's preferred command language interpreter.” Since the devcontainer cannot know what the user's preferred command language interpreter is, this should not be set at all. bash may be a reasonable default as a login shell if one is not otherwise specified (and that's the default in Debian and Ubuntu anyway), but that should be able to be done without setting SHELL.

Normally, I would send a patch to fix this, but you have a CLA, and I don't sign CLAs. So I'm filing this issue so it can be appropriately fixed.