allow setting datadir with env var

The environment variable loader converts the variable names to lowercase,
which prevents the DATADIR from being settable through this mechanism.

This patch stops lowercasing selected variables to allow changing them.

common.php

@@ -31,9 +31,18 @@
 
 // overwrite config with ENV variables
 $env_prefix = $f3->get('env_prefix');
-foreach ($f3->get('ENV') as $key => $value) {
-    if (strncasecmp($key, $env_prefix, strlen($env_prefix)) == 0) {
-        $f3->set(strtolower(substr($key, strlen($env_prefix))), $value);
+foreach ($f3->get('ENV') as $envKey => $value) {
+    // if variable name starts with $env_prefix
+    if (strncasecmp($envKey, $env_prefix, strlen($env_prefix)) == 0) {
+        $key = substr($envKey, strlen($env_prefix)); // remove the prefix
+
+        // most keys are lowercase but there are some exceptions and
+        // we still want to be able to set them using environment variable
+        if (!in_array($key, ['BASEDIR', 'DATADIR'])) {
+            $key = strtolower($key);
+        }
+
+        $f3->set($key, $value);
     }
 }