Dockerfile and fix for protobuf error

[ToSMaverick]

When you try to build and run the package at the moment you will get an error like this:

TypeError: Descriptors cannot not be created directly.
If this call came from a _pb2.py file, your generated code is out of date and must be regenerated with protoc >= 3.19.0.
If you cannot immediately regenerate your protos, some other possible workarounds are:
 1. Downgrade the protobuf package to 3.20.x or lower.
 2. Set PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION=python (but this will use pure-Python parsing and will be much slower).

In order to fix this and to let the script run in a Docker container I used this Dockerfile (inspired by Python on DockerHub):

FROM python:3
WORKDIR /usr/src/meshtastic-mqtt
COPY . ./
RUN pip install protobuf==3.20.1 && pip install .
CMD [ "meshtastic-mqtt" ]