Add Dockerfile & build/push task (#316)

* add dockerfile and actions task to build image

* test on branch

* change to push only on release, update readme

* remove pip install requirements

* change to on release github action

Dockerfile

@@ -0,0 +1,9 @@
+FROM python:3.9-slim
+
+WORKDIR /app
+COPY parsedmarc/ parsedmarc/
+COPY README.rst setup.py ./
+
+RUN python setup.py install
+
+ENTRYPOINT ["parsedmarc"]