custom gitlab-runner-helper image cannot be pulled from private container registry

I am using the GitLab Runner Helm Chart for Kubernetes together with a local GitLab instance.

I am using a custom gitlab-runner-helper image, in order to add self-signed certificates to the helper image.

The image resides in a private container registry with credentials.

When a pipeline is started, the GitLab Runner starts a runner pod that consists of the running image and the custom gitlab-runner-helper. However, the pod is unable to pull the helper image.

ERROR: Job failed (system failure): prepare environment: image pull failed: Back-off pulling image "my.registry.de/gitlab-runner-helper:x86_64-2ebc4dc4". Check https://docs.gitlab.com/runner/shells/index.html#shell-profile-loading for more information

The pod has a imagePullSecret that includes the gitlab registry credentials.

imagePullSecrets:
  - name: runner-apnn5iso-project-917-concurrent-0zm8b8

The pod also includes the serviceAccount: default

However, the default serviceAccount has the required imagePullSecret!

apiVersion: v1
imagePullSecrets:
- name: my-registry
kind: ServiceAccount
metadata:
  name: default
  namespace: gitlab-runner
secrets:
- name: default-token-qwsh7

Is there a way to add the my-registry imagePullSecrets to the Pod directly?