Diagrid dashboard run command in docs panics on Linux

**Describe the issue**
When I follow the steps in [getting started guide](https://docs.dapr.io/getting-started/install-dapr-selfhost/) on a Linux machine and run `docker run -p 8080:8080 ghcr.io/diagridio/diagrid-dashboard:latest` it panics:
```bash
$ docker run -p 8080:8080 ghcr.io/diagridio/diagrid-dashboard:latest
time="2026-04-02T07:39:53.048040523Z" level=info msg="Can not load statestore from localhost, trying docker internal network" instance=acd7d08d96a8 scope=diagrid.dashboard.statestore type=log ver=unknown
panic: redis store: error connecting to redis at host.docker.internal:6379: dial tcp: lookup host.docker.internal on 192.168.1.254:53: no such host

goroutine 1 [running]:
github.com/diagridio/diagrid-dashboard/cmd.Run()
/app/cmd/app.go:24 +0x4b4
main.main()
/app/main.go:6 +0xf
```
Seems like the dashboard tries to connect to Redis on host using `host.docker.internal` DNS name, but this [only resolves on Mac/Windows](https://stackoverflow.com/questions/24319662/from-inside-of-a-docker-container-how-do-i-connect-to-the-localhost-of-the-mach), not Linux.

**URL of the docs**
https://docs.dapr.io/getting-started/install-dapr-selfhost/

**Expected content**
On Linux run:
```bash
docker run --add-host=host.docker.internal:host-gateway -p 8080:8080 ghcr.io/diagridio/diagrid-dashboard:latest
```
(this works as expected)

Start with the getting started guide at docs.dapr.io/getting-started/install-dapr-selfhost/ and find the Diagrid dashboard Docker command. Update the Linux instructions to include the host-gateway mapping shown in the issue, then verify the command matches the expected content and avoids the reported host.docker.internal lookup failure.

Written by the indexing model from the issue text.