[Integration] Add support for Docker runner

[pierrehilbert]

Issue

Describe the enhancement:

Some users are running Elastic Agent on Docker and we are not currently able to tests our different scenarios on this environment.

Describe a specific use case for the enhancement or feature:

To support testing on Docker.

Definition of done

• Create a OSRunner for Docker
• Update the OS support list for the runner
• Ensure that already in place tests are working on Dockers

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)

[ycombinator]

@blakerouse Would we be able to implement this issue by implementing a new InstanceProvisioner?

elastic-agent/pkg/testing/common/instance.go

Lines 46 to 47 in 3fe5741

	// InstanceProvisioner performs the provisioning of instances.
	type InstanceProvisioner interface {

[cmacknz]

I think the kind runner probably covers this, unless we specifically want to test containers separate from k8s.

[cmacknz]

(I don't think there's much value in testing docker separate from k8s)

[ycombinator]

Thanks @cmacknz. Closing for now.