Test image before pushing to registry

This extends the CI to have multiple jobs:
- build
- test
- deploy

The first job builds the image and pushes it to the repo registry (https://gitlab.com/DasSkelett/nginx-quic-docker/container_registry) for use in later steps.
The building is split into a separate script (`build.sh`) that in theory should also be runnable during development on one's own system.
The second job runs the newly built image as a service and uses [this external Docker image](https://hub.docker.com/r/ymuski/curl-http3) with curl with http3 support to make sure that nginx actually listens for HTTP/3 connections.

The third job finally tags and pushes the already built image to Dockerhub.

Fixes #1

See merge request DasSkelett/nginx-quic-docker!1

To mitigate

> /bin/sh: eval: line 116: syntax error: unexpected "&&"