[Feature Request] Systemwide Configuration Support

[anglesideangle]

What

This is a feature request around adding system-wide configuration functionality. For example, a user would be able to write a devcontainer spec-compliant configuration in $XDG_CONFIG_HOME/devcontainer/devcontainer.json that would require ghcr.io/devcontainers-extra/features/neovim-apt-get:1, and that configuration would be merged (same logic as with metadata in containerfile labels) with any project configuration to install neovim in every opened project. Another example of something users might want to put in this file is the podman-specific configuration that the docs for the vscode extension provide.

Why

Currently, users attempting to use tools in a command line environment with the devcontainer cli must add their preferred tools to the project's devcontainer.json. This is often impractical when contributing to large projects, as the maintainer(s) must be convinced to add additional features to their project's devcontainer.json which are not beneficial to every other contributor. It would be ideal to allow users to work with their personal development environments without imposing them on a project.

[RadxaYuntian]

Additionally, if we can specify common options like --docker-path podman in a configuration file, that will help the standalone CLI usage without using VS Code. Right now I'm debugging a container build issue, and VS Code's console cannot fit the full log, so I need to build from command line to tee to a log file.

[anglesideangle]

I've done some further work on this problem and am closing the issue because I don't believe it's possible to install features into arbitrary environments given the way devcontainer features work. Therefore, this functionality would make it relatively easy for users to break their devcontainer cli by making it attempt to install features into incompatible containers.