Azure DevOps self-hosted agent container private registry

I'm trying to run a container job that uses an image from a private registry (not Docker Hub) which requires authentication. When I run the job on an Azure-hosted agent the "Initialize containers" step performs a docker login using the configured Service Connection, but if I switch it to a self-hosted agent, it skips the docker login and goes straight to docker pull, causing the job to fail.

How can I tell it to authenticate against the registry on my self-hosted agent?

EDIT:

This is code in the pipeline:

jobs:
- job: Deployment
  timeoutInMinutes: 120
  cancelTimeoutInMinutes: 5 
  pool:
     name: Azure Pipelines

   container:
     image: ***:latest
     endpoint: <SERVICE CONNECTION NAME>

This is the log from the Azure hosted agent:

Starting: Initialize containers
DockerActionRetries variable value: True
/usr/bin/docker version --format '{{.Server.APIVersion}}'
'1.41'
Docker daemon API version: '1.41'
/usr/bin/docker version --format '{{.Client.APIVersion}}'
'1.41'
Docker client API version: '1.41'
/usr/bin/docker ps --all --quiet --no-trunc --filter "label=f760b8"
/usr/bin/docker network prune --force --filter "label=f760b8"
DockerActionRetries variable value: True
/usr/bin/docker login --username "***" --password-stdin https://***
WARNING! Your password will be stored unencrypted in /home/vsts/.docker/config.json.
Login Succeeded
Configure a credential helper to remove this warning. See
https://docs.docker.com/engine/reference/commandline/login/#credentials-store
DockerActionRetries variable value: True
/usr/bin/docker pull ***:latest
latest: Pulling from ***

And here's the log from the same pipeline with the agent changed to a self-hosted agent:

Starting: Initialize containers
DockerActionRetries variable value: True
/usr/bin/docker version --format '{{.Server.APIVersion}}'
'1.43'
Docker daemon API version: '1.43'
/usr/bin/docker version --format '{{.Client.APIVersion}}'
'1.43'
Docker client API version: '1.43'
/usr/bin/docker ps --all --quiet --no-trunc --filter "label=085311"
/usr/bin/docker network prune --force --filter "label=085311"
DockerActionRetries variable value: True
/usr/bin/docker pull ***/installer-pre-reqs:latest
Error response from daemon: Head "https://***/v2/installer-pre-reqs/manifests/latest": unauthorized: authentication required, visit https://aka.ms/acr/authorization for more information.
##[warning]Docker pull failed with exit code 1, back off 00:00:10 seconds before retry.

In the case of the self-host agent it goes straight to doing the pull and never does the login step.

This is more of a guess than actual knowledge, but the documentation states (concerning login problems with docker hub for multiple container jobs) that:

"The container job uses the underlying host agent Docker config.json for image registry authorization"

So in addition to the other requirements (for linux-based self-hosted agents):

• Ensure Docker is installed
• The agent must have permission to access the Docker daemon

I would guess might need to have pre-created the login information in config.json.

https://learn.microsoft.com/en-us/azure/devops/pipelines/process/container-phases?view=azure-devops

Strangely, creating a new pipeline, using the same YAML, actually resolves this. The docker login now gets called. I can't find anything wrong the original pipeline, even with the "View Full YAML" option