Both Dockers and VMs have different areas of the appliance. Dockers are used when you talk about running the same operating system in the guest and host. So, you can say docker is OS-level virtualization which consumes fewer resources. Whereas VMs are more universal but they have larger overhead.
In order to learn docker and have a certificate in Docker then you can have a look at the following Docker Training Course. If you like to read then you can read the following tutorial on Docker. To watch the free video tutorials on Docker you can visit the below-mentioned website:-
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