Provide a way for programs to know they're running inside a devcontainer/envbuilder workspace

[mafredri]

To allow #68 to work seamlessly there needs to be a way of informing programs that they're running inside a devcontainer.

Example use-case: A coder module (e.g. code-server) runs a startup/install script, sees DEVCONTAINER=true, looks for the devcontainer.json to access .extensions and .settings from customizations.vscode.

Where to look for this devcontainer.json will depends on #211.

This can be accomplished via e.g. environment variables (ENVBUILDER=true, DEVCONTAINER=true). Where to look for the file can also be an env (e.g. DEVCONTAINER_CONFIG=/.devcontainer.json).