adding Racket to the languages for the icfp2020 competition

[Stephen De Gabrielle]

I’ve just cargo-cult’ed a couple of PR’s to add racket to the languages for the icfp2020 competition - I have no idea if they are right so if anyone knows anything about docker I’d appreciate it if they took a look at my PR’s!  https://github.com/icfpcontest2020/dockerfiles/issues/63

all details are linked in the issue

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