Add render_mode attribute to gym class, add optional splash screen, f… #226

Job	Run time
Generate build matrix	12s
build-and-push-arena-images (images/base, base-on3.10-bullseye, on=3.10-bullseye)	1m 46s
build-and-push-arena-images (images/base, base-on3.9-bullseye, on=3.9-bullseye)	1m 59s
build-and-push-arena-images (images/base, base-on3.7-bullseye, on=3.7-bullseye)	2m 16s
build-and-push-arena-images (images/ray_rllib, ray-rllib-on3.10-bullseye, on=3.10-bullseye)	31m 54s
build-and-push-arena-images (images/ray_rllib, ray-rllib-on3.9-bullseye, on=3.9-bullseye)	31m 37s
build-and-push-arena-images (images/stable-baselines, stable-baselines-on3.7-bullseye, on=3.7-bul...	8m 0s
build-and-push-arena-images (images/stable-baselines3, stable-baselines3-on3.10-bullseye, on=3.10...	26m 26s
build-and-push-arena-images (images/stable-baselines3, stable-baselines3-on3.9-bullseye, on=3.9-b...	28m 51s
	2h 13m 1s

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