Add Docker smoke test running init command in CI

[evgeny-stakewise]

Summary

Adds a smoke test to the Docker CI workflow to verify the built container image is functional beyond just building successfully.

Details

In the per-platform docker job, after the push-by-digest build:

• Load image locally for smoke test — rebuilds the image with load: true (reusing the GHA cache, so effectively free) and tags it operator-smoke-test:ci so it can be run locally on the runner.
• Smoke test Docker image — runs the init command inside the container. init exercises the CLI wiring and Python startup without needing execution/consensus RPC endpoints, so it runs offline and exits 0 on success. --no-verify skips the interactive mnemonic verification that would otherwise block on stdin.

If the container fails to start or the CLI is broken, docker run returns non-zero and the step (and job) fails.