feat: docker support

[zogot]

This add a Dockerfile to provide this as a docker image.

This will probably need some extra from your end if you want to publish this to Dockerhub as then an addition could be made to the README.md.

Final Docker Image size:

179MB - 175MB of that is node:16-slim

Reasons for specific things in the Dockerfile:

1. Why npm install --production instead of npm ci in 'productionDependencies' stage.

A comparison with the size of node_modules from npm ci vs npm install --production:

Command	Size of node_modules
npm ci	250M
npm install --production	4.7M

Can check with docker run -it --entrypoint=/bin/bash waitehr:ci if you build with making the changes to the Dockerfile to use the different commands

2. Why tini and what for?

This is for proper handling of signals. Without tini the yargs command doesnt terminate correctly. I think it would be possible to have proper handling of signals but might be a more significant code change.

With tini the listening for SIGINT works as expected.