Adding in Cloudbuild documentation

[annez]

Thought it'd be good to add in Google Cloudbuild. I've also upgraded the referred image on GitLab as that image doesn't seem to work with Lighthouse 6.

We're running on HIGHCPU_32 as it delivers the best and most consistent results (and HIGHCPU_8) is similar to Github Actions which doesn't return consistent results.

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[annez]

@googlebot I signed it!

[patrickhulce]

thanks very much for the addition and the missed update to the docker image! :)

[patrickhulce]

thanks for helping out @annez! 🎉