[Feature] Added a possibility to use a private IP instead of a public IP

[primez]

Hello,

First of all, thank you for such a great tool!

I decided to implement support for using private IPs because I needed this feature and noticed that the related feature request at #809 has been open for a month with no replies. I assumed it might not be a priority or not planned at all.

Disclosure: I am a .NET developer, and my knowledge of Golang is limited. I relied heavily on ChatGPT to help me write this code. However, I have verified its functionality in a container and a Linux environment, and it works as expected, binding a private IP to a domain name.

To make this work, I added a new value to the environment variables:

 environment:
      - PUBLICIP_FETCHERS=privateip

[MRDGH2821]

If run in bare metal, is the private IP in 192.168.X.Y ?

If run inside container, does it give the same private IP as bare metal?

I haven't run the code, but I'll check it out soon & see if both match or not.

(Bare metal means directly installing on host system)

BTW, thanks for this PR!
I hope it will get accepted!

[primez]

A bare metal installation will give you an IP in the range of 192.168..., as well as a VM installation.
In a container, well, I think it depends on a network. I haven't tested this extensively, but in the simplest case, it will give the IP of a container, not the host system.
So, I installed it as a Linux service that starts with the host OS.