Add Docker support for RSSingle, and a workflow for builds

This commit adds a Dockerfile, .dockerignore, and a GH Action for
pushing container images to - and this is by default, other registries
can be used - Docker Hub (you need to generate a token and install it in
the repo), and GitHub Registry, which doesn't require a manual token.

The workflow won't build until #3 is merged, as it relies on successful
builds from that workflow before a container image is pushed. Once we use unit
tests, this will ensure container images aren't broken when pushed to a
remote registry.

I have marked this PR as draft for that reason.

Signed-off-by: Dom Rodriguez <shymega@shymega.org.uk>

Dockerfile

@@ -0,0 +1,18 @@
+FROM docker.io/python:3.8-buster AS base
+
+FROM base AS build
+
+WORKDIR /work
+
+COPY . .
+
+RUN pip3 install pyinstaller
+
+RUN pip3 install -r /work/requirements.txt
+RUN pyinstaller --onefile /work/rssingle.py
+
+FROM docker.io/python:3.8-buster AS app
+
+COPY --from=build /work/dist/rssingle /rssingle
+
+ENTRYPOINT ["/rssingle"]