GitHub Packages requires auth even for pulling, so it's not ideal for a public registry.

Co-authored-by: Teoh Han Hui <teohhanhui@gmail.com>

Preserve DX, as user is likely to just enter "localhost" in the browser's address bar

```
h2-proxy_1     | 2019/06/16 13:33:45 [emerg] 1#1: host not found in upstream "mercure" in /etc/nginx/conf.d/default.conf:76
h2-proxy_1     | nginx: [emerg] host not found in upstream "mercure" in /etc/nginx/conf.d/default.conf:76
sonda_h2-proxy_1 exited with code 1

```

* User YAML merge keys to avoid ducplications

* Remove useless whitespace

Previous links point to a 'this section has moved ...' sentence.

And actually reuse previously built images in CI

Remove ./ relative path notation which doesn't work on latest version of Docker + Compose

See: https://docs.docker.com/compose/compose-file/#dockerfile