move build files into the builds/ folder, rename Dockerfile to also have the stack name

Dockerfile → builds/Dockerfile.heroku-16

@@ -10,4 +10,4 @@ RUN apt-get update && apt-get install -y python-pip && rm -rf /var/lib/apt/lists
 COPY requirements.txt /app/
 RUN pip install --disable-pip-version-check --no-cache-dir -r /app/requirements.txt
 
-COPY . /app
+COPY . /app/builds/