added remote-env arg

[drwaltman]

Added parser argument --remote-env with alias -R when calling retro-contest run to allow for any docker image to be used for the remote container, instead of only openai/retro-env. The default remote image is still openai/retro-env.

I wanted to be able to create a dockerfile from openai/retro-env without having to tag over it in order to use retro-contest run.

[endrift]

Thanks!

[drwaltman]

@TheAggregator thanks for collaboration!