Try Hack Me is one of the web sites, where you can learn new skills on interactive way using machines (available 1 in free mode).

Before this room, also it is available another free room on link:"Intro to Containerisation"
One of the new room is "Intro to Docker". Before this room is good to be familiar with Linux Fundamentals, room for this is on the link:Linux fundamentals.

In the room you can learn:

• Basic docker syntax
• Running Your First Container
• Intro to Dockerfiles
• Intro to Docker compose
• Intro to Docker socket
• Practical

Useful post for 50 docker commands you should know and their use cases on link:50 commands for docker