Use docker credentials in the skypilot orchestrator

[stefannica]

Describe changes

When the container registry is linked to a service connector or configured with docker credentials, these should be used to configure the SkyPilot task to fetch the image inside the VM.

Pre-requisites

Please ensure you have done the following:

• I have read the CONTRIBUTING.md document.
• If my change requires a change to docs, I have updated the documentation accordingly.
• If I have added an integration, I have updated the integrations table and the corresponding website section.
• I have added tests to cover my changes.
• I have based my new branch on develop and the open PR is targeting develop. If your branch wasn't based on develop read Contribution guide on rebasing branch to develop.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Other (add details above)

[htahir1]

it looks good ! but i haven't tested it either, thanks for getting rid of my gnarly code

[safoinme]

Looks good

[stefannica]

Successfully tested using two different stacks:

• AWS skypilot + AWS ECR container registry + S3 artifact store (all linked to the same AWS connector)
• AWS skypilot + S3 artifact store (all linked to the same AWS connector) + Google GCR container registry (linked to a GCP connector)

This shows that the Docker credentials are successfully pulled from the service connector and configured inside the SkyPilot node regardless of the credentials origin.

[stefannica]

Using the Skypilot orchestrator with AWS credentials coming Service Connectors still has some problems due to how AWS credentials are transferred by SkyPilot inside the AWS EC2 VM. Details here: skypilot-org/skypilot#2737