Add optional args to DockerContainer class

[wajdijurry]

This PR aims to add ability to add optional arguments to docker run command through DockerContainer class.
Example:

$container = DockerContainer::create($imageName, $containerName)
    ->setOptionalArgs('-it', '-a')
    ->start();

Notes:

1. README.md updated
2. New test case added

[freekmurze]

Thank you!