unauthorized: authentication required on docker push to a different repo

I'm creating two docker images via gitlab-ci from one repository upon pushing them to GitLabs private container registry. The .gitlab-ci.yml is below. myproject is the group name. backend and docs are GitLab projects within this group. Both have the container registry enabled. build_image works OK but build_apidoc_image fails when pushing the image for the apidoc container to the registry with:

docker push myregistry.com/myproject/docs:latest
The push refers to a repository [myregistry.com/myproject/docs]
630816f32edb: Preparing
530d5553aec8: Preparing
c3f640b7175a: Preparing
378f94990a4e: Preparing
77b53f4affd3: Preparing
4b0bab9ff599: Preparing
d1c800db26c7: Preparing
42755cf4ee95: Preparing
4b0bab9ff599: Waiting
d1c800db26c7: Waiting
42755cf4ee95: Waiting
unauthorized: authentication required
ERROR: Build failed: exit code 1

The strange thing is that docker login succeeds:

$ docker login -u gitlab-ci-token -p $CI_BUILD_TOKEN myregistry.com
Login Succeeded
$ docker build -t myregistry.com/myproject/docs -f apidoc/Dockerfile .
Sending build context to Docker daemon 485.4 kB

This gitlab build is triggered from the backend repository. When building and pushing manually to docs it works.

Here's the .gitlab-ci.yml

stages:
  - build

build_image:
  stage: build
  image: docker:git
  services:
    - docker:dind
  script:
    - docker login -u gitlab-ci-token -p $CI_BUILD_TOKEN myregistry.com
    - docker build -t myregistry.com/myproject/backend .
    - docker push myregistry.com/myproject/backend:latest
  only:
    - master

build_apidoc_image:
  stage: build
  image: docker:git
  services:
    - docker:dind
  script:
    - docker login -u gitlab-ci-token -p $CI_BUILD_TOKEN myregistry.com
    - docker build -t myregistry.com/myproject/docs -f apidoc/Dockerfile .
    - docker push myregistry.com/myproject/docs:latest