optimize Docker build

Dockerfile

@@ -2,7 +2,9 @@ FROM python:latest
 
 # Install dependencies.
 ADD requirements.txt /requirements.txt
-ADD main.py /main.py
 RUN pip install -r requirements.txt
 
-CMD ["python", "/main.py"]
+# Copy code.
+ADD main.py /main.py
+
+CMD ["python", "/main.py"]