Use two-phase deployment for prod.shared

[achave11-ucsc]

@hannes-ucsc: "We were recently putting the finishing touches on a previous promotion (rerunning the deploy and deploy_browser jobs) while already preparing the next one (deploying prod.shared from the PR branch). On prod, the shared deployment is done in one step, which means unused Docker images are removed from ECR right after their replacements have been copied. This could prevent us from rerunning the build_image and test job of the most recent pipeline since both jobs depend on images from ECR, but only if the images were also removed from the local image store in the DinD container."

[hannes-ucsc]

Spike to reproduce. On tempdev, delete the base image of the Azul image (python), the BQ emulator image and the ES image from ECR and then rerun the jobs in the most recent pipeline.

[achave11-ucsc]

@hannes-ucsc: "It may take more time to reproduce in tempdev as instructed above then it would take to just modify the PR template check list. I'd like to think about this some more."

[hannes-ucsc]

We'll just implement the CL changes without trying to reproduce the suspected negative impact.