Add Erlang/OTP 28 (currently 28.0-rc2) (#499) #424

Job	Run time
docker (DIR=master)	9m 7s
docker (DIR=master VARIANT=alpine)	6m 55s
docker (DIR=28)	4m 12s
docker (DIR=28 VARIANT=slim)	3m 8s
docker (DIR=28 VARIANT=alpine)	3m 42s
docker (DIR=27)	3m 46s
docker (DIR=27 VARIANT=slim)	2m 51s
docker (DIR=27 VARIANT=alpine)	3m 16s
docker (DIR=26)	13m 20s
docker (DIR=26 VARIANT=slim)	7m 6s
docker (DIR=26 VARIANT=alpine)	7m 54s
docker (DIR=25)	11m 12s
docker (DIR=25 VARIANT=slim)	6m 5s
docker (DIR=25 VARIANT=alpine)	6m 48s
docker (DIR=24)	9m 43s
docker (DIR=24 VARIANT=slim)	6m 5s
docker (DIR=24 VARIANT=alpine)	6m 41s
	1h 51m 51s

Provide feedback