Admin not working in standalone mode

*Description*:

I set up a standalone deployment using containers by following the instructions at https://gateway.envoyproxy.io/docs/tasks/operations/standalone-deployment-mode/

Then I tried exposing port 19000 to get access to the admin interface, but it doesn't seem to work.

>What issue is being seen? Describe what should be happening instead of
the bug, for example: The expected value isn't returned, etc.

```
$ curl --verbose http://0.0.0.0:19000/
* Trying 0.0.0.0:19000...
* Connected to 0.0.0.0 (0.0.0.0) port 19000
> GET / HTTP/1.1
> Host: 0.0.0.0:19000
> User-Agent: curl/8.7.1
> Accept: */*
>
* Request completely sent off
* Empty reply from server
* Closing connection
curl: (52) Empty reply from server
```

*Repro steps*:
> Include sample requests, environment, etc. All data and inputs
required to reproduce the bug.

```yaml
# docker-compose.yaml

services:
envoy-gateway:
image: envoyproxy/gateway:v1.3.2
ports:
- 8888:8888
- 19000:19000
volumes:
- "./volumes/envoy-gateway:/tmp/envoy-gateway"
command:
- "server"
- "--config-path"
- "/tmp/envoy-gateway/standalone.yaml"
```

```yaml
# standalone.conf

apiVersion: gateway.envoyproxy.io/v1alpha1
kind: EnvoyGateway
gateway:
controllerName: gateway.envoyproxy.io/gatewayclass-controller
provider:
type: Custom
custom:
resource:
type: File
file:
paths: ["/tmp/envoy-gateway/config"]
infrastructure:
type: Host
host: {}
logging:
level:
default: debug
extensionApis:
enableBackend: true
```

>**Note**: If there are privacy concerns, sanitize the data prior to
sharing.

*Environment*:
>Include the environment like gateway version, envoy version and so on.

See above

*Logs*:
>Include the access logs and the Envoy logs.

No logs printed