Add git to image build docker step

[KnVerey]

Our cloud build image uploader started failing a week ago with an error about git not being available in the build container. It never has been, so I'm not sure how it worked before if git was required. But I can reproduce the failure locally, and adding git indeed fixes the problem.

My guess as to the cause of this is that we are not controlling the version of the Go alpine image we're building in, and it updated to use go 1.18. With Go 1.18, the build seems to default to trying to populate [BuildInfo](https://pkg.go.dev/debug/buildinfo@master#BuildInfo) including VCS information. The error I got locally is more informative:

#12 13.23 error obtaining VCS status: exec: "git": executable file not found in $PATH
#12 13.23 	Use -buildvcs=false to disable VCS stamping.

We aren't currently/yet using BuildInfo regardless, and if we do we'll want to make sure the VCS information it gets this way is correct.

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