Add Docker-based test infrastructure for e2e tests (#424)

## Summary

- Add `Dockerfile.test`: Python 3.12 image with Claude Code CLI
installed
- Add `scripts/test-docker.sh`: Local script to run tests in Docker
- Add `test-e2e-docker` job to CI workflow that runs the full e2e suite
in a container
- Add `.dockerignore` to speed up Docker builds

## Context

This helps catch Docker-specific issues like #406 where filesystem-based
agents loaded via `setting_sources=["project"]` may silently fail in
Docker environments.

## Local Usage

```bash
# Run unit tests in Docker (no API key needed)
./scripts/test-docker.sh unit

# Run e2e tests in Docker
ANTHROPIC_API_KEY=sk-... ./scripts/test-docker.sh e2e

# Run all tests
ANTHROPIC_API_KEY=sk-... ./scripts/test-docker.sh all
```

## Test plan

- [x] Unit tests pass in Docker locally (129 passed)
- [ ] CI job runs successfully

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude <noreply@anthropic.com>

.github/workflows/test.yml

@@ -81,6 +81,24 @@ jobs:
         run: |
           python -m pytest e2e-tests/ -v -m e2e
 
+  test-e2e-docker:
+    runs-on: ubuntu-latest
+    needs: test # Run after unit tests pass
+    # Run e2e tests in Docker to catch container-specific issues like #406
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Build Docker test image
+        run: docker build -f Dockerfile.test -t claude-sdk-test .
+
+      - name: Run e2e tests in Docker
+        env:
+          ANTHROPIC_API_KEY: ${{ secrets.ANTHROPIC_API_KEY }}
+        run: |
+          docker run --rm -e ANTHROPIC_API_KEY \
+            claude-sdk-test python -m pytest e2e-tests/ -v -m e2e
+
   test-examples:
     runs-on: ubuntu-latest
     needs: test-e2e # Run after e2e tests