Add optional apt-cacher proxy configuration.

Support APT_PROXY build arg in Docker/Compose, document setup, and include a safe .env.example template for local cache usage.

Co-authored-by: Cursor <cursoragent@cursor.com>

.gitignore

@@ -5,6 +5,8 @@ nvidia-smi*
 # local runtime data
 players/*_steam/
 players/*_config/sunshine.conf
+.env
+!.env.example
 
 # do not publish private Sunshine TLS keys
 players/*_config/credentials/