Remove old Dockerfile for Jekyll site

[tfogo]

PR #9128 added a new Dockerfile for the Hugo site. The old
staging container is no longer needed.

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[tfogo]

/assign @MistyHacks

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[mdlinville]

@lucperkins Should you cherry-pick your commit from your other PR into this one, to keep all of this type of work into one PR?

[lucperkins]

@tfogo I agree with @MistyHacks that it'd be best to consolidate the Docker-related changes into a single PR.

[tfogo]

@lucperkins sure thing. @MistyHacks is there a PR from @lucperkins that should be cherry picked into this one or did you mean from my other PR touching the Dockerfile (#9216)?

[chenopis]

/hold
Consolidate the PRs and close the one that's no longer needed.

[tfogo]

This has been consolidated into #9216. Closing this PR.