Set readOnlyRootFileSystem to improve security posture

[AlanGreene]

Changes

Similar change already made in Chains, and PRs open for Pipelines and Triggers /kind misc

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[AlanGreene]

Need to check how this impacts use of nginx config templating in https://github.com/alangreene/dashboard-next. May need to mount a volume for use by some of the config scripts at startup. /hold