'An image does not exist locally with the tag' error in docker PUSH command

Question

I am new to docker. I have the following two docker image files that I downloaded from docker hub

(1) Cassandra:3 
(2) Kong

I have Gitlab Repository into which I want to push these two images. I am able to connect to the Gitlab Registry. I am trying to push the Cassandra:3 Image using the following command.

docker push registry-gitlab.mycompany.com/username/myproject/cassandra:3

When I execute the above command I get the following error.

 The push refers to repository [registry-gitlab.mycompany.com/username/myproject/cassandra]
An image does not exist locally with the tag: registry-gitlab.mycompany.com/username/myproject/cassandra

There is an image called cassandra with tag 3 in the system where I am executing the push command. After going through similar questions in stackoverflow, I assume that this relates to Tag name, but I am not sure what should be the tag name to be assigned to this image or whether updating tags will have any issue.

Answer 1

First you have to tag the docker image,

docker tag Cassandra:3 registry-gitlab.mycompany.com/username/myproject/cassandra:3

Then you can push

docker push registry-gitlab.mycompany.com/username/myproject/cassandra:3

Answer 2

I've tagged my repo. My gitlab docker gives me a 404. I guess I'm with tagging it wrong or have my group/project/image wrong.