No instruction on docker images

### Your use case

Following [Self Host Guide](https://github.com/element-hq/element-call/blob/livekit/docs/self-hosting.md), I've already set up the matrix backend (`synapse, eturnal, livekit and livekit-jwc`) and they work fine on element-web / element desktop using element-call feature.

> The only blocker in the guidance is the `lk-jwt-service` -> env `LIVEKIT_URL=wss://matrix-rtc.mydomain.com/livekit/sfu/`, I thought it should be a local address `wss://127.0.0.1:` and struggled for a long time

I've already provided `element-web` service as `chat.mydomain.com` and it works fine. Now I just want to get the standalone version of `element-call` working as well to support instant calling.

Therefore, I want to deploy `element-call` via the released docker image using my self-hosted server.

#### What would you like to do?
Add guidance on how to use the released docker images.

I currently use following compose file
```yaml
services:
element-call:
image: ghcr.io/element-hq/element-call:v0.16.0
environment:
- LIVEKIT_KEY=
- LIVEKIT_SECRET=
- VITE_DEFAULT_HOMESERVER=https://matrix.mydomain.com/
ports:
- 8080:8080
```
(According to https://github.com/element-hq/element-call/issues/2220#issuecomment-1966706130, it seems those environment variables are no longer needed?)

I use nginx to reverse proxy port 8080 to `call.mydomain.com` with tls certs, it gets stuck at loading screen with following errors in console:
```
A preload for 'https://call.mydomain.com/config.json' is found, but is not used because the request credentials mode does not match. Consider taking a look at crossorigin attribute.

rageshake.ts:495 Failed to load config SyntaxError: Unexpected token '<', "