docs: user ARG to chose image, install python3 before running pip3

docs/Dockerfile

@@ -1,12 +1,13 @@
-FROM python:3-alpine
+ARG DISTRO=alpine:3.8
+FROM $DISTRO
 
 COPY requirements.txt /requirements.txt
 
 ARG version=master
 ENV VERSION=$version
 
-RUN pip install -r /requirements.txt \
- && apk add --no-cache nginx curl \
+RUN apk add --no-cache nginx curl python3 \
+ && pip3 install -r /requirements.txt \
  && mkdir /run/nginx
 
 COPY ./nginx.conf /etc/nginx/conf.d/default.conf