Replace pia with gluetun wherever possible

- in Readme documentation
- Changed splash title string
- Changed Dockerfile labels
- Changed commands and docker-compose service & container name

docker-compose.yml

@@ -1,8 +1,8 @@
 version: "3.7"
 services:
-  pia:
+  gluetun:
     image: qmcgaw/private-internet-access
-    container_name: pia
+    container_name: gluetun
     cap_add:
       - NET_ADMIN
     network_mode: bridge