fix: preserve Docker feature flags when enabling CDI

[git-jxj]

Description

Running nvidia-ctk runtime configure --runtime=docker --enable-cdi against a daemon.json with existing feature flags replaces them with only cdi: true. For example, containerd-snapshotter: true and buildkit: false disappear.

Docker configuration is decoded into map[string]any, but EnableCDI asserts the features entry as map[string]bool. Use the decoded map type so existing settings survive while CDI is enabled.

Add file-based tests for missing/empty features, existing true and false flags, CDI already enabled or disabled, and repeated load/update/save operations.

Checklist

• No secrets, sensitive information, or unrelated changes
• Unit tests passing (make test) — the full suite has the baseline failures described below.
• Lint checks passing (make lint)
• Test cases are added for new code paths
• Commits are signed-off and cryptographically signed.

Testing

• Three preservation cases fail before the fix and pass afterward.
• go test -race ./pkg/config/... ./cmd/nvidia-ctk/runtime/... -count=1 passed.
• make lint: 0 issues.
• The real CLI preserves existing features and unrelated settings in a temporary daemon.json.

make test has the same four failing packages and nine failing subtests on unmodified upstream and this branch: installer, installer/toolkit, modifier, and nvcdi. These involve missing test driver libraries and CSV hook expectation differences; the full suite is not reported as passing.

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[henry118]

/ok to test e3184b6