Use pip to install dependencies in Dockerfile

Dockerfile

@@ -3,6 +3,7 @@ FROM python:3.7-slim AS builder
 WORKDIR /tmp/repo
 COPY setup.py requirements.txt /tmp/repo/
 COPY nnfwtbn /tmp/repo/nnfwtbn
+RUN pip install -r requirements.txt
 RUN python setup.py install
 
 FROM python:3.7-slim