Gitlab :how to push custom docker image to different repo/namespace

I have two repositories, let's call them project1 and project2. I can successfully build an image as project1_image from project1 and push it to the project1 container registry using a pipeline job in project1. However, when I try to build an image as project2_image and push it to the project2 container registry from project1's pipeline, I get the following error:

"denied: requested access to the resource is denied" when attempting to push.

Is it possible to create an image during project1's pipeline, name it project2_image, and push it to project2's container registry?

I am using gitlabs default runners.

Here is the .gitlab-ci.yml file in project1 that is attempting to push the image to project2.

build image:
    stage: build
    image: docker:stable
    services:
        - name: docker:dind
          alias: thedockerhost
    variables:
        # Tell docker CLI how to talk to Docker daemon; see
        # https://docs.gitlab.com/ee/ci/docker/using_docker_build.html#use-docker-in-docker-executor
        DOCKER_HOST: tcp://thedockerhost:2375/
        # Use the overlayfs driver for improved performance:
        DOCKER_DRIVER: overlay2
        DOCKER_TLS_CERTDIR: ""    
    script:
            - docker login -u "$CI_REGISTRY_USER" -p "$CI_REGISTRY_PASSWORD" "$CI_REGISTRY"
            - docker build -t gitlab.com:5050/groupname/project2/pythonapp .
            - docker push gitlab.com:5050/groupname/project2/pythonapp

Is it possible to create an image during project1's pipeline, name it project2_image, and push it to project2's container registry?

Yes.

"denied: requested access to the resource is denied" when attempting to push.

So use proper credentials. Auto supplied -u "$CI_REGISTRY_USER" -p "$CI_REGISTRY_PASSWORD" are for accessing project1, not project2.