Update Dockerfile - Githubissues

Suggest to refactor the Dockerfile:

base the image on python:3.7-slim instead of python:3.7-alpine
when doing that, there's no need for build tools as there's binary versions of required python packages
as we're not installing the build tools, the resulting image size is smaller
this also makes the docker image build process much faster (xml lib build take a while)
there's no need for entrypoint.sh, updated ENTRYPOINT to just run python fire.py
updated Dockerfile should be run from the cloned repo - it just copies files, doesn't do git clone

ustayready / fireprox