Add ability to override any configs with env

Use environemnt variables to override any value in config.php

Also, locked version on the ubuntu image to something that actually
builds.

Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu
+FROM ubuntu:14.04
 MAINTAINER Christian Lück <christian@lueck.tv>
 
 RUN DEBIAN_FRONTEND=noninteractive apt-get update && apt-get install -y \

README.md

@@ -164,6 +164,18 @@ For more information check out the [official documentation](https://github.com/g
 -e SELF_URL_PATH=https://example.org/ttrss
 ```
 
+### Additional configuration
+
+In addition to the environement variables above, it is possible to override any value in the `config.php`
+by using another environment variable. Prefix the config value you want to override with `CONFIG_`
+and it will replace the default value on container creation.
+
+For example, to add the `auth_external` plugin, set the following variable:
+
+```
+-e CONFIG_PLUGINS='auth_internal, note, auth_external'
+```
+
 ### Testing ttrss in foreground
 
 For testing purposes it's recommended to initially start this container in foreground.

configure-db.php

@@ -100,6 +100,15 @@ catch (PDOException $e) {
     unset($pdo);
 }
 
+$config_prefix = 'CONFIG_';
+foreach ($_SERVER as $name => $value) {
+    if (strpos($name, $config_prefix) === 0) {
+        $name = substr($name, strlen($config_prefix));
+        echo 'Getting config from env: ' . $name . PHP_EOL;
+        $config[$name] = $value;
+    }
+}
+
 $contents = file_get_contents($confpath);
 foreach ($config as $name => $value) {
     $contents = preg_replace('/(define\s*\(\'' . $name . '\',\s*)(.*)(\);)/', '$1"' . $value . '"$3', $contents);