make 28 as latest (#505) #440

Job	Run time
docker (DIR=master)	9m 13s
docker (DIR=master VARIANT=alpine)	6m 39s
docker (DIR=28)	3m 58s
docker (DIR=28 VARIANT=slim)	3m 10s
docker (DIR=28 VARIANT=alpine)	3m 20s
docker (DIR=27)	3m 44s
docker (DIR=27 VARIANT=slim)	2m 49s
docker (DIR=27 VARIANT=alpine)	3m 8s
docker (DIR=26)	14m 44s
docker (DIR=26 VARIANT=slim)	6m 52s
docker (DIR=26 VARIANT=alpine)	8m 21s
docker (DIR=25)	24s
docker (DIR=25 VARIANT=slim)	6m 9s
docker (DIR=25 VARIANT=alpine)	6m 45s
docker (DIR=24)	9m 27s
docker (DIR=24 VARIANT=slim)	5m 55s
docker (DIR=24 VARIANT=alpine)	6m 35s
	1h 41m 13s

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