adding the docker utility to checks
adding the docker utility to checks

According to the DC/OS documentation, docker must be installed on all nodes including the bootstrap nodes. This check is will ensure that docker still is running and still is installed. https://docs.mesosphere.com/1.11/installing/oss/custom/system-requirements/#docker
dcos/dcos,mesosphere-mergebot/mergebot-test-dcos,surdy/dcos,mesosphere-mergebot/dcos,surdy/dcos,kensipe/dcos,GoelDeepak/dcos,dcos/dcos,GoelDeepak/dcos,mesosphere-mergebot/mergebot-test-dcos,mesosphere-mergebot/dcos,mesosphere-mergebot/mergebot-test-dcos,dcos/dcos,dcos/dcos,kensipe/dcos,mesosphere-mergebot/mergebot-test-dcos,surdy/dcos,kensipe/dcos,mesosphere-mergebot/dcos,GoelDeepak/dcos,dcos/dcos,mesosphere-mergebot/dcos,kensipe/dcos,surdy/dcos,GoelDeepak/dcos