Allow 'custom' (subdirectory) .devcontainer.json to be started

[gbraad]

🌈 Feature

At the moment only the default .devcontainer/devcontainer.json is used, but this in my case points to .devcontainer/default/devcontainer.json. I have multiple setup definitions, as is possible with Codespaces. Since the default is used, this causes a problem on startup, as at the moment #8752 prevents a start. An alternative would be to allow a more 'barebones' defintion, or allow a default (configurable) path to consider, like .devcontainer/codesandbox/ if exists.