Q:

Magento 2 Not Building Product Thumbnails Cache

There are other, similar questions out there but non of which have the correct answer for me. 
I have a fresh Magento 2 installation, built using composer.  I have done nothing on it apart from install sample data using the php bin/magento sampledata:deploy command followed by setup:upgrade, setup:di:compile and setup:static-content:deploy. 
When I go to a category page, the images are all 404-ing.  The folder pub/media/catalog/product/cache is not being generated.  The permissions are as per the Magento 2 docs:
pub/media/catalog/product$ ll
drwxrwsrwx  8 matt     www-data 4096 Apr 21 10:38 ./
drwxrwsr-x  3 matt     www-data 4096 Apr 20 14:02 ../
drwxrwsr-x 35 www-data www-data 4096 Apr 21 10:35 cache/
drwxrwsrwx  3 matt     www-data 4096 Apr 20 14:02 g/
drwxrwsrwx  4 matt     www-data 4096 Apr 20 14:02 l/
drwxrwsrwx  9 matt     www-data 4096 Apr 20 14:02 m/
drwxrwsrwx  4 matt     www-data 4096 Apr 20 14:02 u/
drwxrwsrwx  9 matt     www-data 4096 Apr 20 14:02 w/

I have GD installed and am running PHP 5.6. 
The memory limit in my php.ini file is set to 2048M. I know this is far too high but I was ruling out the memory limit as a cause. 
I have reinstalled Magento 2 several times. 
I have created PHP script to create folders and files within the product cache directory and that works without issues. 
When I go to a product view page or run php bin/magento catalog:images:resize, the pub/media/catalog/product/cache folder is generated. 
I have no idea what to do from here!  Let me know if you need anything else from me. 
Would really appreciate any kind of help on this! 
For reference, here is a list of enabled PHP modules:
[PHP Modules]
calendar, Core, ctype, curl, date, dom, ereg, exif, fileinfo, filter, ftp, gd, gettext, hash, iconv, intl, json, libxml, mbstring, mcrypt, mhash, mysql, mysqli, mysqlnd, openssl, pcntl, pcre, PDO, pdo_mysql, Phar, posix, readline, Reflection, session, shmop, SimpleXML, soap, sockets, SPL, standard, sysvmsg, sysvsem, sysvshm, tokenizer, wddx, xml, xmlreader, xmlwriter, xsl, Zend OPcache, zlib

[Zend Modules]
Zend OPcache

A:

So I finally found the issue tracked on GitHub for the Magento 2 repo here
The only solution (for now) is to modify your composer.json file and change any references for 2.1.6 to 2.1.5.  For example:
{
  "name": "magento/project-community-edition",
  "description": "eCommerce Platform for Growth (Community Edition)",
  "type": "project",
  "version": "2.1.5", // Here
  "license": [
    "OSL-3.0",
    "AFL-3.0"
  ],
  "require": {
    "magento/product-community-edition": "2.1.5", // Here
    "composer/composer": "@alpha",
    "bitexpert/magento2-force-customer-login": "1.3.1",
    "magento/data-migration-tool": "2.1.5" // Here
  },
...

Then run composer update. 
As always when changing Magento 2 versions, fun the following commands:

php bin/magento setup:upgrade
php bin/magento setup:di:compile
php bin/magento setup:static-content:deploy

It's a little annoying that this is a problem caused by the Magento dev team but keep an eye on the issue linked above.  I am sure they will realease a fix soon. 
Happy Magentoing!