Enable http2 Support for nginx with docker

[t-book]

HTTP2 the successor of HTTP 1.1 brings advantages in speed as it lifts the limitation of concurrent requests to infinite.

HTTP 1.1

Firefox 2:  2
Firefox 3+: 6
Opera 9.26: 4
Opera 12:   6
Safari 3:   4
Safari 5:   6
IE 7:       2
IE 8:       6
IE 10:      8
Chrome:     6

For a comparison of loading have a look here: https://imagekit.io/demo/http2-vs-http1?utm_source=blog&utm_medium=blog&utm_campaign=Blog

For further information about HTTP1.1 vs HTTP2 visit:

• https://css-tricks.com/http2-real-world-performance-test-analysis/#:~:text=HTTP%2F2%20is%20faster%20and,site%20loading%20time%20than%20HTTP1.
• https://imagekit.io/blog/http2-vs-http1-performance/

As Browsers will accept HTTP2 only with a secure transport protocol only changes to nginx.https.available.conf.envsubst are needed as shown here: https://docs.google.com/document/d/1fsFA1Zq1ZSYMETIFEEtquB9AlhNHNHuM3_ZSZhLK4gg/edit?usp=sharing

(The support of IPv6 is optional but could be an improvement as well)