Core

On certain server setups, setting ApplicationContext fails, because the environment variable TYPO3_CONTEXT is never set.

The reason is that variables set by mod_rewrite get prefixed with REDIRECT_
This seems to appear in PHP-FPM setups, which is quite popular these days.

htaccess file:
 RewriteCond %{HTTP_HOST} ^dev\.example\.com$
 RewriteRule (.*) $1 [E=TYPO3_CONTEXT:Development]

Test:
echo $_SERVER['TYPO3_CONTEXT'];

echo $_SERVER['REDIRECT_TYPO3_CONTEXT'];
Development

Flow solves this by using REDIRECT_FLOW3_CONTEXT as fallback in index.php:

$context = getenv('FLOW3_CONTEXT') ?: (getenv('REDIRECT_FLOW3_CONTEXT') ?: 'Development');

We should backport this fallback to Bootstrap.