Feature: GitHub workflow

[carlomion]

Fixes #15.
Add GitHub workflow to:

• build the docker image, caching the intermediate layers to speed up the CI pipeline
• test that all the containers start up successfully
  • setup the DB with monaco OSM data
  • test that /health endpoint returns succesfully

[dsperling]

Adding workflows in welcome, but a few requests:

1. This project is focused on small Docker images, but the build-time optimizations make the n7m app image larger. Can we back these changes out without impacting the workflow?
2. We are splitting n7m-app into two images - n7m-api and n7m-feed. These changes will be merged soon and the workflow will need to be changed. See feature/api branch.
3. Could you recommend text for the README for developers to get this setup?