Support of "GroupAdd" configuration.

### Summary
Docker client support option `--group-add`
or
API parameter
```
"HostConfig": {
...
"GroupAdd": ["newgroup"],
...
}
```

Would be nice to be able to set this parameter from the TaskDefinition. Support of this configuration was introduced in the [API v1.20](https://docs.docker.com/engine/api/v1.20/)

Start with the requested TaskDefinition configuration and compare it with Docker's --group-add option and the API v1.20 HostConfig example linked in the issue. Determine where TaskDefinition options are documented or implemented, and consider the work complete when GroupAdd can be specified there and passed through as the requested configuration.

Written by the indexing model from the issue text.