Accessing Prive Docker Repository for Service Image

I am trying to access a docker image for a service in a bitbucket build. The build fails with the following message:

rpc error: code = Unknown desc = failed to resolve image "docker.io/mydockerhubaccount/mycustomapp:latest-release": no available registry endpoint: pull access denied, repository does not exist or may require authorization: server message: insufficient_scope: authorization failed

The image resides in a private docker hub repository, and I have configured the 'custom-app' service as follows:

...
definitions:
  services:

    postgres:
      image: postgres:11.5
      variables:
        POSTGRES_DB: 'postgres'
        POSTGRES_USER: 'somePassword'

    custom-app:
      image: mydockerhubaccount/mycustomapp:latest-release
      username: $DOCKER_HUB_USER
      password: $DOCKER_HUB_PASSWORD
      variables:
        ...  

The username and password are valid and configured correctly as BitBucket Repository variables (we use the same variables in the same pipeline to push images to the same private docker hub repository). I have also confirmed that the image is available on the docker hub repo by pulling it independently on my development machine. 

Is there something missing in my configuration that would cause this error?