Continuous Integration: Build public and ready to use docker image

[bjoern-hempel]

• Version build process
  • $ bin/console app:version:bump
    • ./VERSION
  • ./api/config/packages/api_platform.yaml (version: '0.0.0')
  • $ git tag -a "v0.1.5" -m "version v0.1.5"
  • $ git push origin "v0.1.5"
  • etc.
• CI process
  • composer test
  • Build docker image
  • Upload docker image to docker hub