switch opam ci to docker #279

Job	Run time
coq	1m 6s
coq-elpi	1m 0s
hierarchy-builder	1m 5s
mathcomp-ssreflect	1m 3s
mathcomp-bigenough	1m 8s
mathcomp-finmap	1m 5s
mathcomp-fingroup	1m 1s
mathcomp-algebra	1m 2s
mathcomp-classical	1m 8s
mathcomp-solvable	1m 3s
multinomials	1m 2s
mathcomp-field	3m 16s
mathcomp-reals	2m 24s
mathcomp-experimental-reals	1m 7s
mathcomp-reals-stdlib	1m 6s
mathcomp-real-closed	1m 6s
mathcomp-analysis	10m 6s
mathcomp-character	1m 1s
mathcomp	7m 7s
mathcomp-analysis-stdlib	17m 25s
odd-order	11m 12s
	1h 7m 33s

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