## 🐛 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

Capabilities stereotype = JSON.toType(allConfigs.get(i), Capabilities.class);
      kinds.put(imageName, stereotype);
    }
    
    
    
    String hostname = HostIdentifier.getHostName();
    Optional<ContainerInfo> info = docker.inspect(new ContainerId(hostname));
    DockerAssetsPath assetsPath = getAssetsPath(info);
    String networkName = getDockerNetworkName(info);
    loadImages(docker, kinds.keySet().toArray(new String[0]));
    Image videoImage = getVideoImage(docker);
    loadImages(docker, videoImage.getName());
    int maxContainerCount = Runtime.getRuntime().availableProcessors();
    ImmutableMultimap.Builder<Capabilities, SessionFactory> factories = ImmutableMultimap.builder();
    kinds.forEach((name, caps) -> {
      Image image = docker.getImage(name);
      for (int i = 0; i < maxContainerCount; i++) {
        factories.put(
          caps,
          new DockerSessionFactory(
            tracer,
            clientFactory,
            docker,
            getDockerUri(),
            image,
            caps,
            videoImage,
            assetsPath,
            networkName,