Update dependency wpackagist-theme/twentysixteen to v2.6

This MR contains the following updates:

Package	Type	Update	Change
wpackagist-theme/twentysixteen (source)	require	minor	2.5 -> 2.6

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Dockerfile

-FROM composer:1.10.1 as build
+FROM composer:2.2.9 as build
 
 ADD . /build
 WORKDIR /build