Devcontainers: support user-defined `devcontainer.json` that's not checked into git

Opening this as a potential use-case, but we can wait for customer feedback.

Ultimately I think there will be a use-case for users to want to override the project `devcontainer.json` without causing local changes.

The issue describes only a potential use case: allowing a user-defined devcontainer.json to override the project configuration without local changes. Start by confirming the customer need and reviewing how the project devcontainer.json is discovered or loaded; done criteria are not defined in the issue and require clarification.

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