Add jitter to Docker release builds (#20021)
## Summary
- add a random 0–10 second delay before versioned Docker release builds
enter the release environment gate
- leave pull request dry-runs and `uv-dev` builds unchanged
This staggers the extra-image matrix jobs and reduces the chance of race
conditions in the release environment gate.
## Test plan
- `uvx prek run prettier --files .github/workflows/build-docker.yml`
- `uvx zizmor .github/workflows/build-docker.yml`
- `git diff --check`
