Build Tools: Switch all Travis PHPUnit jobs over to Docker-based PHP.

Additionally, there are a handful of related minor changes in this commit:
- `.travis.yml` has been tidied up a little.
- [45745] was incorrectly marking Travis jobs as passed when some PHPUnit runs failed.
- Add the `LOCAL_PHP_MEMCACHED` environment variable, for enabling Memcached.
- Add the `env:pull` NPM script, for refreshing Docker images.

See #47767.


git-svn-id: https://develop.svn.wordpress.org/trunk@45762 602fd350-edb4-49c9-b593-d223f7449a82

tools/local-env/docker-compose.scripts.yml

@@ -13,6 +13,7 @@ services:
 
     environment:
       LOCAL_PHP_XDEBUG: ${LOCAL_PHP_XDEBUG-false}
+      LOCAL_PHP_MEMCACHED: ${LOCAL_PHP_MEMCACHED-false}
 
     volumes:
       - ../../:/var/www
@@ -31,6 +32,7 @@ services:
 
     environment:
       LOCAL_PHP_XDEBUG: ${LOCAL_PHP_XDEBUG-false}
+      LOCAL_PHP_MEMCACHED: ${LOCAL_PHP_MEMCACHED-false}
 
     volumes:
       - ./phpunit-config.ini:/usr/local/etc/php/conf.d/phpunit-config.ini

tools/local-env/docker-compose.yml

@@ -38,6 +38,7 @@ services:
 
     environment:
       LOCAL_PHP_XDEBUG: ${LOCAL_PHP_XDEBUG-false}
+      LOCAL_PHP_MEMCACHED: ${LOCAL_PHP_MEMCACHED-false}
 
     volumes:
       - ./php-config.ini:/usr/local/etc/php/conf.d/php-config.ini