fix to Dockerfile install flask

Dockerfile

@@ -2,15 +2,14 @@ FROM python:3.8-slim-buster as builder
 RUN python -m venv /opt/venv
 RUN apt-get update && apt-get install -y build-essential
 COPY requirements.txt .
-RUN pip install uwsgi
-RUN pip install -r requirements.txt
+RUN pip3 install uwsgi
+RUN pip3 install -r requirements.txt
 
 FROM python:3.8-slim-buster
 COPY --from=builder /opt/venv  /opt/venv 
 ENV PATH="/opt/venv/bin:$PATH"
-RUN pip install flask
 WORKDIR /app/
 
 COPY . .
-RUN ls
+RUN pip3 install flask
 CMD flask run