RunDockerAction should respect default CMD unless cmd parameter is not specified

Due to the line below, default `CMD` stated in Dockerfile is always overridden:

https://github.com/aws/amazon-ssm-agent/blob/master/agent/plugins/dockercontainer/dockercontainer.go#L192
```
commandArguments = append(commandArguments, pluginInput.Image)
commandArguments = append(commandArguments, pluginInput.Cmd)
```

Since `pluginInput.Cmd` is not a required parameter, this plugin should handle it like as other optional inputs, for example:

```
commandArguments = append(commandArguments, pluginInput.Image)
if len(pluginInput.Cmd) > 0 {
commandArguments = append(commandArguments, pluginInput.Cmd)
}
```

Otherwise, the customers always need to specify the same value as Dockerfile.

Start in agent/plugins/dockercontainer/dockercontainer.go around line 192, where the Docker command arguments are assembled. Verify how an omitted Cmd affects the image's default Dockerfile CMD, and confirm that an explicitly provided Cmd still overrides it. Done means RunDockerAction preserves the default CMD when Cmd is empty.

Written by the indexing model from the issue text.