[ci.jenkins.io] Set up an ECR pull through cache

[dduportal]

During the last summer, we had to setup a Docker "Pull Through" caching registry in Azure to avoid builds breaking due to HTTP/429 rate limit from DockerHub: #4192 (comment)

Same can be done on AWS with ECR: https://docs.aws.amazon.com/AmazonECR/latest/userguide/pull-through-cache.html

Moving ci.jenkins.io to AWS needs the same kind setup:

• Registry MUST be private (AWS privatelink) as it cannot be using authentication through Docker Engines (might be solved using EC2 instance identity though)
• Only DockerHub need to be synced
• We need for VMs (with Docker CE). Nice to have for the EKS cluster (when autoscaling)