Updates to use image based on 7.2-cli-alpine3.8
Updates to use image based on 7.2-cli-alpine3.8

Unfortunately, I'm running into the same problems I had with 3.7 images:

```text
Generating blog feeds
=====================

 Generating base feeds
    0 [░░░░░░░░░░░░░░░░░░░░░░░░░░░░][31-Oct-2018 15:51:49 America/Chicago] PHP Warning:  DOMDocument::loadXML(): xmlParseEntityRef: no name in Entity, line: 167 in /var/www/vendor/zendframework/zend-feed/src/Writer/Renderer/Entry/Atom.php on line 404
[31-Oct-2018 15:51:49 America/Chicago] PHP Fatal error:  Uncaught TypeError: Argument 1 passed to DOMDocument::importNode() must be an instance of DOMNode, null given in /var/www/vendor/zendframework/zend-feed/src/Writer/Renderer/Entry/Atom.php:371
Stack trace:
  #0 /var/www/vendor/zendframework/zend-feed/src/Writer/Renderer/Entry/Atom.php(371): DOMDocument->importNode(NULL, 1)
  #1 /var/www/vendor/zendframework/zend-feed/src/Writer/Renderer/Entry/Atom.php(53): Zend\Feed\Writer\Renderer\Entry\Atom->_setContent(Object(DOMDocument), Object(DOMElement))
  #2 /var/www/vendor/zendframework/zend-feed/src/Writer/Renderer/Feed/Atom.php(91): Zend\Feed\Writer\Renderer\Entry\Atom->render()
  #3 /var/www/vendor/zendframework/zend-feed/src/Writer/Feed.php(237): Zend\Feed\Writer\Renderer\Feed\Atom->render()
  #4 /var/www/src/Blog/Console/FeedGenerator.php(209): Zend\Feed\Writer\Feed->export('Atom')
  #5 /var/www/src/Blog/Console/FeedGenerator.php(145): Mwop\Blog\Console\FeedGenerator->generateFeed('atom', 'data/feeds/', 'https://mwop.ne...', 'Blog entries ::...', 'blog', 'blog.feed', Array, in /var/www/vendor/zendframework/zend-feed/src/Writer/Renderer/Entry/Atom.php on line 371
Script bin/mwop.net.php blog:feed-generator handling the build:blog event returned with error code 255
Script @build:blog was called via docker:site
```

This "just works" under the ubuntu-based image, but not in either the
alpine 3.7 or 3.8 images.

weierophinney/mwop.net,weierophinney/mwop.net,weierophinney/mwop.net,weierophinney/mwop.net