Update 16.0.Dockerfile for Google search

What does this MR do and why?

We need to have the GOOGLE_SEARCH_KEY environment variable available during the build process to get Google search working on the 16.0 site.

While we did backport the required changes in !3872 (merged) and !3876 (merged), we also need to update 16.0's Dockerfile to match the update in single.Dockerfile (which is just a template).

We can see in the last build that GOOGLE_SEARCH_KEY was not consumed:

[Warning] One or more build-args [GOOGLE_SEARCH_KEY] were not consumed

This should clear up that warning and provide 16.0 with the env var when building in Docker.

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