Add full IP chain for getting real IP of user in the element UI.

[TafkaMax]

Hi, haven't done a PR in a while. I've noticed that the IP users see in element does not match their computer IP.

https://paigekim29.medium.com/understanding-x-forwarded-for-header-settings-in-nginx-4929f49d57dd

Instead of using $remote_addr which is the last known address, which might usually be from a proxy, I changed it to use the full chain instead.

---

I also added the option to use the synapse version that is defined in the arg SYNAPSE_VERSION

[realtyem]

Not completely sure why the worker's mode tests failed yet; I retriggered them to run again. If they fail again I'll investigate more

I'm onboard with the header adjustment, that looks like what I thought it was supposed to be doing.

The SYNAPSE_VERSION dockerfile change though, I'm not sure how that will interfere with the build-context docker argument found places like this that I use to guarantee I'm getting the exact local image I want to use. It may do nothing, it may override it. Any insight on how this works?

[TafkaMax]

I can remove it here and just set the nginx opts. I used the synapse_version arg in docker compose to locally build against specific version of synapse with the nginx fixes.

[TafkaMax]

Removed the SYNAPSE_VERSION.