docs(lab03): complete documentation

Author: ostxxp

app_python/docs/LAB03.md

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+# LAB03 — CI/CD with GitHub Actions
+
+## 1. Overview
+
+- Testing framework: **pytest**
+  - Chosen for simple syntax and good Flask integration.
+- Linting tool: **ruff**
+  - Fast Python linter, easy to integrate in CI.
+- CI: GitHub Actions
+  - Runs on push and pull request for `app_python/**`
+- Versioning strategy: **Calendar Versioning (CalVer)**
+  - Format: `YYYY.MM.<run_number>`
+  - Also tags image as `latest`
+- Docker image:
+  - `<your-dockerhub-username>/devops-info-service`
+
+---
+
+## 2. Local Testing
+
+Run locally:
+
+```bash
+cd app_python
+pip install -r requirements.txt
+ruff check .
+pytest -q
+```
+
+Example output:
+
+`3 passed in 0.24s`
+
+---
+
+## 3. Workflow Evidence
+
+- ✅ Tests and lint pass in GitHub Actions
+    
+- ✅ Docker image built and pushed automatically
+    
+- ✅ Tags created:
+    
+    - `YYYY.MM.<run_number>`
+        
+    - `latest`
+        
+- ✅ CI status badge added to README
+    
+
+---
+
+## 4. CI Best Practices Implemented
+
+- **Dependency caching** (`cache: pip`)  
+    Speeds up repeated workflow runs.
+    
+- **Job dependency (`needs`)**  
+    Docker build runs only if tests and lint pass.
+    
+- **Path filters**  
+    Workflow runs only when `app_python/**` changes.
+    
+- **Concurrency cancel**  
+    Cancels outdated runs on same branch.
+    
+
+---
+
+## 5. Key Decisions
+
+### Why CalVer?
+
+This project is a service (not a library), so breaking changes are not critical for version communication.  
+CalVer makes versioning simple and automatically generated in CI.
+
+### Docker Tags
+
+- `YYYY.MM.<run_number>`
+    
+- `latest`
+    
+
+### What is tested?
+
+- `GET /`
+    
+- `GET /health`
+    
+- 404 error handling
+    
+- JSON response structure