update dockerfile and default config

[piccione99]

1. updated default config so we don't need a config file for running docker
2. updated readme instructions
3. added an example config file for reference
4. couldn't get entrypoint.sh to work so currently not using it

[denopink]

the idea makes sense since it's a pattern we already put into practice for our services, i.e.: the service should be able to start with the default config.

However, do we want that to be true for xmidt-agent since it's running on a cpe? Not sure if that's a desired pattern for software running on cpe.

Approving this, but I think we shouldn't merged until @schmidtw chimes in.

[schmidtw]

Yes, in general we want the app to basically work right out of the box. We also want it to be secure out of the box, too. I will review this later; there are a few parts being built into the configuration that I think we should do via environment variables (the device_id and other identity items being at the top of this list).