Add Dockerfile and usage readme

docker/Dockerfile

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+FROM alpine:latest
+
+RUN apk add --update --no-cache --virtual .build-deps  \
+    git
+
+RUN apk add --update --no-cache \
+    fontconfig \
+    py3-pillow \
+    py3-future \
+    py3-jinja2 \
+    py3-bottle \
+    python3 \
+    ttf-dejavu \
+    py3-pip \
+    poppler-utils
+
+RUN ln -s /usr/bin/python3 /usr/bin/python
+RUN git clone --depth=1 https://github.com/tbnobody/brother_ql_web.git
+
+WORKDIR /brother_ql_web
+
+RUN rm -r .git
+RUN python3 -m venv .venv
+RUN source .venv/bin/activate
+RUN pip3 install -r requirements.txt
+RUN apk del .build-deps
+
+EXPOSE 8013
+
+CMD [ "./run.py", "--model", "QL-580N", "tcp://192.168.5.169" ]

docker/README.md

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+## Running via docker
+
+First build the docker image and name it `brother_ql_web`, which only needs doing once.
+
+1. `cd docker`
+1. `docker build -t brother_ql_web .`
+
+    > _don't miss the dot at the end!_
+
+To run the image:
+
+1. `docker run -it --rm -p 127.0.0.1:8013:8013 brother_ql_web ./run.py --model <MODEL> <FILE>`
+
+    > replacing <MODEL> with your printers model, and <FILE> with your printers connection  file/network e.g. `tcp://192.168.5.169` or `file:///dev/usb/lp0`
+1. Open http://localhost:8013/ in your web browser