FIX: Move add-on downloads out of temp

Temp may is space-restricted on SSP and so a hidden folder in there
runs out of disk. This sets up a new folder on the mounted EBS. Making
it a hidden pathname prevents web access.

.platform.yml

@@ -8,3 +8,9 @@ crons:
   queuedjobs_task:
     time: '* * * * *'
     command: 'test -f /var/www/mysite/www/host.txt && /usr/local/bin/sera 1 php /var/www/mysite/www/framework/cli-script.php dev/tasks/ProcessJobQueueTask 2>&1 | logger -t SilverStripe_cron'
+shared_dirs:
+  mysite:
+    "addon-downloads": {}
+  url_rules:
+    mysite:
+      '^/addon-downloads/': 'deny'

README.md

@@ -20,6 +20,11 @@ from [Packagist](http://packagist.org).
 
 ## Dependencies
 
+### Environment variables
+
+ * `SS_ADDONS_DOWNLOAD_PATH`: Set this to the full path of the folder to download into. Otherwise, a subfolder of the
+   SilverStripe temp path will be used.
+
 ### ElasticSearch
 
 [ElasticSearch](http://www.elasticsearch.org) is used to provide add-on package indexing and searching.

mysite/code/services/AddonBuilder.php

@@ -9,7 +9,7 @@
 class AddonBuilder
 {
 
-    const ADDONS_DIR = 'add-ons';
+    const ADDONS_DIR = 'addon-downloads';
 
     const SCREENSHOTS_DIR = 'screenshots';
 
@@ -46,9 +46,13 @@ public function build(Addon $addon)
                 continue;
             }
 
-            $path = implode('/', array(
-                TEMP_FOLDER, self::ADDONS_DIR, $addon->Name
-            ));
+            if (defined('SS_ADDONS_DOWNLOAD_PATH')) {
+                $path = SS_ADDONS_DOWNLOAD_PATH . '/' . $addon->Name;
+            } else {
+                $path = implode('/', array(
+                    TEMP_FOLDER, self::ADDONS_DIR, $addon->Name
+                ));
+            }
 
             // Convert PackagistAPI result into class compatible with Composer logic
             $package = new Package(