Added support for quic + http/3

Please read https://github.com/macbre/docker-nginx-brotli#quic--http3-support on how to set up nginx and your browser to talk using quic (http/3).

docker pull macbre/nginx-brotli:1.19.6-http3

(note the -http3 label suffix)

• downgrade to nginx 1.19.6 ⚠️ in order to have http/3 support patch work properly
• use the most recent curl to test http/3 support in our CI / Docker build pipeline
• tested with Firefox 88

See #35 and #32 for more details.