I think it's just because you're on ubuntu trusty/LTS which was released in 2014. They're only including security updates to avoid breaking changes, so nginx is frozen at version 1.4 which was the current version at the time.
Ubuntu vivd has nginx 1.6.2, wily includes nginx 1.9.3. If you used ppa:nginx/stable, you'd get nginx 1.8 which doesn't have http2 support (it was introduced in 1.9), but does add some other useful features which aren't in 1.4 such as weak etags for gzipped content. ppa:nginx/development (which tracks nginx mainline versions) does include http2.
The nginx release notes are here: http://nginx.org/en/CHANGES
If the nginx version is updated, http2 won't be on by default. It's only enabled if "http2" is added to your listen directive(s), for example "listen 443 default_server ssl http2;".
I believe there is a breaking change between nginx 1.4 and 1.9 with the default configs. The line "include /etc/nginx/fastcgi_params;" needs to be changed to "include /etc/nginx/fastcgi.conf;". To avoid any impact for users of the container, after performing the update /etc/nginx/fastcgi_params could be symlinked to /etc/nginx/fastcgi.conf.
edit: If the repositories are changed, it's probably best to change to ppa:nginx/stable, not development. That doesn't have http2 yet. It tracks official nginx stable releases.