fix: game-ci test --docker defaulted to an image that can't actually run tests

[frostebite]

Root cause found via set -x tracing (#99) on a real CI failure: every Docker test run on unity-test-runner's thin-wrapper PR (#310) exited near-instantly with exit code 1 and zero log output, even though activation succeeded fine moments earlier in the same run.

Root cause

--docker defaulted targetPlatform to NoTarget, resolving to RunnerImageTag's base editor module — the same image activate/build's NoTarget path already uses successfully. But activation only needs the editor to launch and exit; running tests needs it to actually compile and execute test assemblies, which base apparently can't do (no Unity log output was ever written at all, meaning it failed before even initializing far enough to open the log file).

unity-test-runner's own original ImageTag never defaulted to NoTarget — it defaulted to this host's native Standalone target (getImagePlatformType: StandaloneLinux64 on Linux, StandaloneWindows on Windows), which resolves to the linux-il2cpp module for Unity 2020+. Matching that default here instead.

Also in this PR

• Removes the temporary set -x tracing from runsteps.sh (debug: trace runsteps.sh execution to diagnose a real CI failure #99) now that it's served its purpose.
• The default now reads from options.hostPlatform (already resolved per-invocation) rather than raw process.platform directly in executeDocker — was also just untestable, not only stylistic.

Testing

• Full bun test ./src: 156 pass, 0 fail (updated one test's assertion from the old, wrong base expectation to the correct linux-il2cpp one).
• bun build succeeds.
• Will re-verify against a real Docker test run once this lands and a release is cut.

🤖 Generated with Claude Code

[coderabbitai]

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