Add Docker-based test infrastructure for e2e tests

Add Docker test infrastructure to catch container-specific issues like #406
where filesystem-based agents may silently fail in Docker environments.

Changes:
- 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
- Add .dockerignore to speed up Docker builds

Usage:
  ./scripts/test-docker.sh unit    # Run unit tests in Docker
  ./scripts/test-docker.sh e2e     # Run e2e tests (needs ANTHROPIC_API_KEY)
  ./scripts/test-docker.sh all     # Run all tests

🤖 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