Our pipeline keeps on failing without giving a reason why.

It certainly has something to do with our custom Docker image, but I cannot figure out what exactly. Whenever I try another public image, the pipeline runs just fine. But if I use our own, it fails on the build step.
The Docker image: https://hub.docker.com/r/sudwestfryslan/default-pipeline
bitbucket-pipelines.yml
image:
name: sudwestfryslan/default-pipeline
username: $DOCKER_HUB_USERNAME
password: $DOCKER_HUB_PASSWORD
email: $DOCKER_HUB_EMAIL
pipelines:
default:
- step:
script:
- php --version
- npm --version
What I have tried:
- Changing to another public image -> Works! Unfortunately, that's not a solution as I want to use a custom image.
- Include the tag in the image name (e.g. :latest or :php74) -> makes no difference
- Include the complete URL to the docker registry -> makes no difference
- Include/exclude docker credentials -> makes no difference
Right now I'm kind of stuck, as I do not know what error is occurring.