Feature Request: Support for Global Environment Variables / Secrets

Currently, when using Dev Containers locally, there is no native way to inject global environment variables or secrets across multiple projects. While GitHub Codespaces provides a "Secrets" feature that allows users to define environment variables once and have them available in every Codespace, local Dev Containers require hardcoding variables in devcontainer.json.

This becomes particularly problematic when using dotfiles. Many developers use dotfiles to automate their environment setup, but dotfiles often require API keys, tokens, or private configurations (e.g., ONEPASSWORD_TOKEN, AGE_PRIVATE_KEY, etc.). To keep these secure, we cannot commit them to the dotfiles repository.

I would like to propose a way to define Global Environment Variables in VS Code that are automatically injected into any Dev Container session.