Define a Docker health check

### Feature Description

I'd like to use Docker Compose's [health checks](https://docs.docker.com/compose/compose-file/05-services/#healthcheck) to bring up the proxy before starting dependent services, but I can't seem to be able to ping the readiness endpoints from within the container image.

It would be awesome if there was a health check defined in the cloud-sql-proxy image itself (see https://docs.docker.com/engine/reference/builder/#healthcheck).

### Sample code

_No response_

### Alternatives Considered

I tried pinging the existing readiness endpoint:
```
services:
...
cloud_sql_proxy:
...
healthcheck:
test: ['CMD', 'curl', '-f', 'http://localhost:9999/readiness']
interval: 3s
timeout: 3s
retries: 5
```

However, it seems that `curl` is not available in the Docker image, and neither is any other utility that would allow one to hit the readiness endpoint.

### Additional Details

_No response_