Update Dockerfile for smaller image size

- Smaller image size isn't everything, but when you can shrink the operational image size by 60% its an obvious decision.
- Update to Python 3.8

Dockerfile

@@ -1,16 +1,25 @@
-FROM python:3.7.4-alpine3.10
+FROM python:3.8-alpine3.10 as compile
 RUN mkdir /app
 WORKDIR /app
 RUN apk add --no-cache curl
 RUN curl -O -fssL https://github.com/grpc-ecosystem/grpc-health-probe/releases/download/v0.3.0/grpc_health_probe-linux-amd64 && \
     mv grpc_health_probe-linux-amd64 /usr/local/bin/grpc_health_probe && \
     chmod 755 /usr/local/bin/grpc_health_probe
 RUN apk add --no-cache libstdc++
+RUN python -m venv /opt/venv
+ENV PATH="/opt/venv/bin:$PATH"
 COPY requirements.txt /app
 RUN apk add --no-cache --virtual build-deps build-base && \
     pip install --no-cache-dir -r requirements.txt && \
     apk del build-deps
+
+FROM python:3.8-alpine3.10
+RUN apk add --no-cache libstdc++
+RUN mkdir /app
+WORKDIR /app
 COPY src/ /app/
+COPY --from=compile /opt/venv /opt/venv
+ENV PATH="/opt/venv/bin:$PATH"
 RUN python -m grpc_tools.protoc \
     -I. \
     --python_out=. \