cache in /var/cache

this should be configurable

Dockerfile

@@ -2,4 +2,5 @@ FROM ulsmith/alpine-apache-php7
 
 COPY ./ /app/public/
 
-RUN chown -R apache:root /app/public
+RUN mkdir -p /var/cache/rss-bridge/ && \
+        chown -R apache:root /app/public /var/cache/rss-bridge/

index.php

@@ -4,7 +4,7 @@ require_once __DIR__ . '/lib/RssBridge.php';
 define('PHP_VERSION_REQUIRED', '5.6.0');
 
 // Specify directory for cached files (using FileCache)
-define('CACHE_DIR', __DIR__ . '/cache');
+define('CACHE_DIR', '/var/cache/rss-bridge');
 
 // Specify path for whitelist file
 define('WHITELIST_FILE', __DIR__ . '/whitelist.txt');