Add Docker plugin test #556

Job	Run time
integration-tests / Get runner image / Get runner image	5s
integration-tests / Build and push charm	2m 31s
integration-tests / Build rock / Get rocks	3s
integration-tests / Build image / Get images	2s
integration-tests / Build rock / Build rock (jenkins_rock)	4m 31s
integration-tests / Build image / Build image	0s
integration-tests / Get rocks or Docker images	1s
integration-tests / Integration tests / Integration tests (test_jenkins.py)	16m 55s
integration-tests / Integration tests / Integration tests (test_k8s_agent.py)	16m 44s
integration-tests / Integration tests / Integration tests (test_machine_agent.py)	25m 32s
integration-tests / Integration tests / Integration tests (test_plugins.py)	30m 52s
integration-tests / Integration tests / Integration tests (test_proxy.py)	12m 40s
integration-tests / Integration tests / Integration tests (test_cos.py)	24m 37s
integration-tests / Required Integration Test Status Checks	1s
	2h 14m 34s

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