change rc tag name (#500) #427

Job	Run time
docker (DIR=master)	9m 10s
docker (DIR=master VARIANT=alpine)	6m 46s
docker (DIR=28)	4m 12s
docker (DIR=28 VARIANT=slim)	3m 9s
docker (DIR=28 VARIANT=alpine)	3m 30s
docker (DIR=27)	3m 43s
docker (DIR=27 VARIANT=slim)	2m 48s
docker (DIR=27 VARIANT=alpine)	3m 1s
docker (DIR=26)	12m 28s
docker (DIR=26 VARIANT=slim)	7m 3s
docker (DIR=26 VARIANT=alpine)	7m 32s
docker (DIR=25)	11m 13s
docker (DIR=25 VARIANT=slim)	6m 14s
docker (DIR=25 VARIANT=alpine)	6m 39s
docker (DIR=24)	9m 36s
docker (DIR=24 VARIANT=slim)	5m 49s
docker (DIR=24 VARIANT=alpine)	6m 42s
	1h 49m 35s

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