switch opam ci to docker #270

Job	Run time
coq	1m 5s
coq-elpi	1m 27s
hierarchy-builder	0s
mathcomp-ssreflect	0s
mathcomp-bigenough	0s
mathcomp-finmap	0s
mathcomp-fingroup	0s
mathcomp-algebra	0s
mathcomp-solvable	0s
multinomials	0s
mathcomp-classical	0s
mathcomp-field	0s
mathcomp-reals	0s
mathcomp-character	0s
mathcomp-real-closed	0s
mathcomp-analysis	0s
mathcomp-experimental-reals	0s
mathcomp-reals-stdlib	0s
mathcomp	0s
mathcomp-analysis-stdlib	0s
odd-order	0s
	2m 32s

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