Unfortunately it was not that simple. Linux alpine was using libressl which does not yet support TLS1.3, however with Alpine 3.9 they have switched to OpenSSL 1.1.1a which does support TLS1.3. In addition, main docker nginx repo has just rebased their image to Alpine 3.9, officially supporting TLS1.3!!!
@aptalca Please update your repo to alpine 3.9 as this will allow everyone to enable TLS1.3! 😄