## 🐛 Bug Report
There is no way to provide max-sessions parameter in the docker section of the standalone config file


## To Reproduce

My grid config looks as follows. Is there a way to limit the number of sessions available for both Firefox and Chrome in the docker section of standalone config file
```[toml]
[node]
detect-drivers = false
max-sessions=4

[docker]
configs = ["selenium/standalone-chrome:latest", "{\"browserName\": \"chrome\", \"browserVersion\": \"91\"}", "selenium/standalone-firefox:latest", "{\"browserName\": \"firefox\", \"browserVersion\": \"89\"}"]
host = "localhost"
port = 2375
url = "unix:/var/run/docker.sock"
video-image = "selenium/video:latest"
```

Running this command `java -jar selenium-server-4.0.0-beta-4.jar standalone --config grid.config.toml` creates 12 sessions each of Chrome and Firefox with a max concurrency of 4

![image](https://user-images.githubusercontent.com/17963268/126071586-743c2f2a-3ed3-4299-b8bf-1512c70d1f12.png)

In the file https://github.com/SeleniumHQ/selenium/blob/941dc9c6b2e2aa4f701c1b72be8de03d4b7e996a/java/server/src/org/openqa/selenium/grid/docker/DockerOptions.java#L137

 we have initialised it to the # of availableProcessors

whereas in the file https://github.com/SeleniumHQ/selenium/blob/941dc9c6b2e2aa4f701c1b72be8de03d4b7e996a/java/server/src/org/openqa/selenium/grid/node/config/NodeOptions.java#L79

we take into consideration the value available in the node section of the config file

## Expected behavior

There should be a config to set max-sessions in the docker section

## Test script or set of commands reproducing this issue

## Environment

Selenium Grid version (if applicable): selenium-server-4.0.0-beta-4

}
      return new URI(DEFAULT_DOCKER_URL);
    } catch (URISyntaxException e) {
      throw new ConfigException("Unable to determine docker url", e);
    }
  }
  private boolean isEnabled(Docker docker) {
    if (!config.getAll(DOCKER_SECTION, "configs").isPresent()) {
      return false;
    }
    
    return docker.isSupported();
  }
  public Map<Capabilities, Collection<SessionFactory>> getDockerSessionFactories(
    Tracer tracer,
    HttpClient.Factory clientFactory) {
    HttpClient client = clientFactory.createClient(ClientConfig.defaultConfig().baseUri(getDockerUri()));
    Docker docker = new Docker(client);
    if (!isEnabled(docker)) {
      throw new DockerException("Unable to reach the Docker daemon at " + getDockerUri());
    }
    List<String> allConfigs = config.getAll(DOCKER_SECTION, "configs")
        .orElseThrow(() -> new DockerException("Unable to find docker configs"));
    Multimap<String, Capabilities> kinds = HashMultimap.create();
    for (int i = 0; i < allConfigs.size(); i++) {
      String imageName = allConfigs.get(i);
      i++;
      if (i == allConfigs.size()) {
        throw new DockerException("Unable to find JSON config");
      }