Add Docker smoke test to Docker Build Check workflow

[rroy676]

Motivation

• Add a CI-level smoke test that verifies the built Docker image actually starts and the HTTP API responds, so pull requests fail when the image is non-functional.
• Run the image locally in the runner (no push) and validate runtime behavior by calling the health endpoint /api/health.

Description

• Update the existing workflow to build the image tagged stationarr:smoke-test with push: false, load: true, and the build arg APP_VERSION=smoke-test so the image is local-only and versioned for testing.
• Add a step that runs the image as a container named stationarr-smoke-test with docker run -d --name stationarr-smoke-test -p 3000:3000 stationarr:smoke-test and retries curl against /api/health for up to 30 seconds.
• Fail the job unless the health endpoint response contains "ok":true, print the health response to logs, dump container logs on timeout/failure, and always stop/remove the test container with docker rm -f stationarr-smoke-test in an if: always() cleanup step.

Testing

• Parsed the modified workflow with ruby -e "require 'yaml'; YAML.load_file('.github/workflows/docker-build-check.yml'); puts 'YAML OK'", which succeeded.
• Ran git diff --check to validate whitespace/format issues, which reported no problems.
• Attempted a local docker build --build-arg APP_VERSION=smoke-test -t stationarr:smoke-test . but the Docker CLI is not available in this environment, so a local image build was not executed here (the smoke test will run in CI where Docker is available).

---

Codex Task