Update/expand the instruction for keeping base images up to date

[DamianSawicki]

Follow-up to #691. Golang imports play nice with dependabot, but kubernetes/dns also requires keeping base images up to date. It would be good to update and expand the relevant section "Vulnerability patching" of README.

[Michcioperz]

I'm happy to document what I've been doing when I find a moment

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/lifecycle stale

[Michcioperz]

/remove-lifecycle stale

I will get to this eventually