Move to multi-stage docker build

Also add additional packages to the build container for hosts that don't know anything about python, like Docker Desktop on OSX/Windows.

Dockerfile

@@ -1,6 +1,15 @@
-FROM python:3.7-alpine
-RUN /sbin/apk add tor
-COPY . /opt/sherlock/
-RUN /usr/local/bin/pip install -r /opt/sherlock/requirements.txt
+FROM python:3.7-alpine as build
+RUN apk add --no-cache linux-headers g++ gcc libxml2-dev libxml2 libxslt-dev
+COPY requirements.txt /opt/sherlock/
+WORKDIR /wheels
+RUN pip3 wheel -r /opt/sherlock/requirements.txt
 
-ENTRYPOINT ["python", "/opt/sherlock/sherlock.py"]
+FROM python:3.7-alpine
+COPY --from=build /wheels /wheels
+COPY . /opt/sherlock/
+WORKDIR /opt/sherlock
+RUN pip3 install -r requirements.txt -f /wheels \
+  && rm -rf /wheels \
+  && rm -rf /root/.cache/pip/*
+
+ENTRYPOINT ["python", "sherlock.py"]