## 🐛 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

}
  private URI getDockerUri() {
    try {
      Optional<String> possibleUri = config.get(DOCKER_SECTION, "url");
      if (possibleUri.isPresent()) {
        return new URI(possibleUri.get());
      }
      Optional<String> possibleHost = config.get(DOCKER_SECTION, "host");
      Optional<Integer> possiblePort = config.getInt(DOCKER_SECTION, "port");
      if (possibleHost.isPresent() && possiblePort.isPresent()) {
        String host = possibleHost.get();
        int port = possiblePort.get();
        if (!(host.startsWith("tcp:") || host.startsWith("http:") || host.startsWith("https"))) {
          host = String.format("http:%s:%s", host, port);
        } else {
          host = String.format("%s:%s", host, port);
        }
        URI uri = new URI(host);
        return new URI(
          "http",
          uri.getUserInfo(),
          uri.getHost(),
          uri.getPort(),
          uri.getPath(),
          null,
          null);
      }
      
      if (Platform.getCurrent().is(WINDOWS)) {
        return new URI("http:localhost:2376");