The course quality is high because Pluralsight has a full-time editing staff that carefully reviews every video produced. They enforce audio, video and content standards that help ensure a consistent and high quality result.
Pluralsight offers a variety of courses on Docker:
Docker Deep Dive
Docker and containers: The Big Picture
First Look: Native Docker Clustering
Integrating Docker with DevOps Automated Workflows
Plural site offers both pre-course and post-course assessments. Passing post-assessments will allow you to view and print off a certificate of achievement.
The official Docker documentation is written in a way that it can act as a tutorial on its own.
Everything is clearly written and directs you to resources if you need further information.
This tutorial is targeted towards people who have never used Docker before.
Those who already have some experience with Docker might be better off with more intermediate resources.
The author of the blog, Ian Miell, has contributed to Docker and has extensive experience building/maintaining Docker-based infrastructure.
Currently he is writing a book on Docker.
The chapters are very compressed and each has 5-15 objectives to reach. Because of its compressed style, it takes very little time to complete a chapter and understand it, making it perfect for learning in your spare time.
Allows zero downtime deployment of new releases on a cluster of machines. So your service can scale, resist even if there is a hardware failure, recover from them, etc.
While learning about Kubernetes will give you a better understanding of Docker, it's not specifically a learning resource.
This is a better route for someone who already has some Docker experience.
Kubernetes configuration and set up requires some more work than just Docker. Using cloud solutions (like Google Container Engine) simplify this, but it's still a bit harder to learn than let's say docker-compose.