Add tracing instrumentation for command execution

[joshrotenberg]

Context

Cross-language parity gap surfaced while comparing docker-wrapper with the Elixir docker_wrapper_ex. The Elixir side has first-class telemetry integration (events around run/build/compose/stream lifecycle); the Rust side currently emits nothing.

Proposal

Add tracing spans to DockerCommand::execute and execute_stream, plus the compose/buildx/swarm feature-gated paths:

• Top-level span per command invocation with fields: command (e.g. run, ps, compose up), args_count, platform (docker/podman/nerdctl when detected).
• tracing::info! on start, tracing::info! on successful exit with duration + exit_code.
• tracing::warn! / error! on non-zero exit with stderr snippet (bounded length).
• Stream variant: tracing::debug! per line under a parent span so callers can filter verbose paths at the subscriber level.

Gate behind an optional tracing feature (default on) so no-instrumentation builds stay lean.

Parity reference

docker_wrapper_ex emits [:docker_wrapper, :exec, :start|:stop|:exception] events with equivalent metadata. Matching fields on the Rust side makes it easy to correlate logs when both wrappers run in the same system (e.g. multi-language test harnesses).

Out of scope

• OpenTelemetry exporter wiring (leave to the consuming binary).
• Metrics -- add later via a metrics crate bridge if demand surfaces.