add support for getting internal IP

Pterodactyl/Panel#613

entrypoint.sh

@@ -1,16 +1,14 @@
 #!/bin/ash
-sleep 5
+sleep 2
 
 cd /home/container
 
+# Make internal Docker IP address available to processes.
+export INTERNAL_IP=`ip route get 1 | awk '{print $NF;exit}'`
+
 # Replace Startup Variables
 MODIFIED_STARTUP=`eval echo $(echo ${STARTUP} | sed -e 's/{{/${/g' -e 's/}}/}/g')`
 echo ":/home/container$ ${MODIFIED_STARTUP}"
 
 # Run the Server
 ${MODIFIED_STARTUP}
-
-if [ $? -ne 0 ]; then
-    echo "PTDL_CONTAINER_ERR: There was an error while attempting to run the start command."
-    exit 1
-fi