feat: add dockerfile for documentation

docs.Dockerfile

+FROM alpine:3.7
+
+ENV PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/root/.local/bin
+
+COPY requirements.txt /mkdocs/
+WORKDIR /mkdocs
+VOLUME /mkdocs
+
+RUN apk --no-cache --no-progress add py-pip \
+  && pip install --user -r requirements.txt