1)

docker tag nginx:latest unix1998/repo_2019:nginx_test1

nginx:latest  is an image in your local server, can be any other 

unix1998/repo_2019:nginx_test1

unix1998 is user id in dcoekr hub,

repo_2019  is repo name created in unix1998 @ docker hub

2)  docker push unix1998/repo_2019:nginx_test1

will be OK