fix: restrict app names to prevent command injection

The previous app name validation regex permitted shell metacharacters such as `;`, `$`, backticks, `|`, and `&`. These names were embedded unquoted into the generated git pre-receive hook script, allowing an authenticated user to execute arbitrary commands as the dokku user simply by pushing to a remote with a crafted app name. App names are now restricted to lowercase alphanumerics, dots, and hyphens, and the hook script also quotes the app variable as a defense-in-depth measure.

plugins/git/internal-functions

@@ -464,7 +464,7 @@ fn-git-create-hook() {
 set -e
 set -o pipefail
 
-cat | DOKKU_ROOT="$DOKKU_ROOT" dokku git-hook $APP
+cat | DOKKU_ROOT="$DOKKU_ROOT" dokku git-hook "$APP"
 EOF
   chmod +x "$PRERECEIVE_HOOK"
 }