Use sqlite3 database as default for Docker image (#2182)

Signed-off-by: Tao Wang <twang2218@gmail.com>

docker/etc/templates/app.ini

@@ -8,7 +8,7 @@ TEMP_PATH = /data/gitea/uploads
 APP_DATA_PATH = /data/gitea
 
 [database]
-HOST = mysql:3306
+DB_TYPE = sqlite3
 PATH = /data/gitea/gitea.db
 
 [session]