make docker build faster (#167)

Author: maxday

tests/Dockerfile

@@ -3,9 +3,15 @@ FROM python:$python_version
 
 ENV PYTHONDONTWRITEBYTECODE True
 
-RUN mkdir -p /test
+RUN mkdir -p /test/datadog_lambda
 WORKDIR /test
 
+# Copy minimal subset of files to make pip install succeed and be cached (next docker builds will be way faster)
+COPY setup.py .
+COPY README.md .
+COPY datadog_lambda/__init__.py datadog_lambda/__init__.py 
+
+RUN pip install .[dev]
+
 # Install datadog-lambda with dev dependencies from local
 COPY . .
-RUN pip install .[dev]