Add render_mode attribute to gym class, add optional splash screen, f… #228

Job	Run time
Generate build matrix	14s
build-and-push-arena-images (images/base, base-on3.10-bullseye, on=3.10-bullseye)	2m 4s
build-and-push-arena-images (images/base, base-on3.9-bullseye, on=3.9-bullseye)	2m 30s
build-and-push-arena-images (images/base, base-on3.7-bullseye, on=3.7-bullseye)	1m 55s
build-and-push-arena-images (images/ray_rllib, ray-rllib-on3.10-bullseye, on=3.10-bullseye)	33m 43s
build-and-push-arena-images (images/ray_rllib, ray-rllib-on3.9-bullseye, on=3.9-bullseye)	32m 32s
build-and-push-arena-images (images/stable-baselines, stable-baselines-on3.7-bullseye, on=3.7-bul...	8m 30s
build-and-push-arena-images (images/stable-baselines3, stable-baselines3-on3.10-bullseye, on=3.10...	28m 29s
build-and-push-arena-images (images/stable-baselines3, stable-baselines3-on3.9-bullseye, on=3.9-b...	25m 18s
	2h 15m 15s

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