Generate content even for container preview

[sftim]

This change makes sure that generated content is up to date when previewing locally in a container.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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[sftim]

I'm going to use this as a basis for my work on #374 (whether we merge it or not).

[sftim]

:thought_balloon: do we always want this behavior, or (eg) only when a particular variable is set?

[jberkus]

What's the status of this PR? It doesn't sound like you're ready with it.

[sftim]

This works for me @jberkus

I don't know whether it's useful for other contributors, and I can't easily answer that question.

~I'll rebase it~ I rebased it against master

[jberkus]

/lgtm