Vendor Python dependencies for offline image builds

Dockerfile

@@ -7,7 +7,8 @@ ENV PYTHONDONTWRITEBYTECODE=1 \
 WORKDIR /app
 
 COPY requirements.txt .
-RUN pip install --no-cache-dir -r requirements.txt
+COPY vendor/ ./vendor/
+RUN pip install --no-cache-dir --no-index --find-links=/app/vendor -r requirements.txt
 
 COPY . .
 RUN mkdir -p /app/pipeline_outputs