Add the latest Docker images to the CI. #66

Job	Run time
build (coqorg/coq:8.9)	4m 32s
build (coqorg/coq:8.12-ocaml-4.10-flambda)	4m 57s
build (coqorg/coq:8.16)	8m 46s
build (coqorg/coq:8.11)	3m 41s
build (coqorg/coq:8.15)	8m 47s
build (coqorg/coq:8.14)	9m 2s
build (coqorg/coq:8.13)	7m 39s
build (coqorg/coq:8.17)	8m 10s
build (coqorg/coq:8.18)	8m 1s
build (rocq/rocq-prover:9.0)	9m 55s
build (coqorg/coq:8.20)	1h 10m 57s
build (rocq/rocq-prover:dev)	8m 2s
build (coqorg/coq:8.19)	1h 7m 6s
	3h 39m 35s

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