Add a preflight check that asserts docker connectivity

[zegl]

Assert that the docker daemon is running, and that we're able to connect to it.
This is done by executing 'docker ps', and checking the exit code.

#39
#554

[k8s-ci-robot]

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[aojea]

I think this should go to pkg/container/docker

[aojea]

/ok-to-test
/assign @neolit123
/assign @BenTheElder

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[APPROVALNOTIFIER] This PR is NOT APPROVED

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[BenTheElder]

Hi, thanks again for the PR!

If you check the current code, you'll see we actually already do a filtered docker ps to list the existing clusters, what we really need is to surface this error better instead of making another call.

I'm going to discuss at today's meeting (in 9 minutes!) and update the preflight checks issue as well to go from pre-flight to post-failure, which I think will help avoid piling on lots of slow checks (not that this one is, but presumably we'd add more) in the normal case, and instead diagnose the environment on failure.

[k8s-ci-robot]

@zegl: PR needs rebase.

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[BenTheElder]

going another route, thanks