Pin Spanner Docker Image Version

[data-sync-user]

We want to simplify the maintenance of Spanner versions in Syncstorage-rs.

Currently, we pull an image for Spanner, pull a version of the Spanner emulator for unit tests, as well as build Spanner using the docker-compose.spanner.yaml for testing. We would like to have a centralized location where this version is defined to prevent possible regressions, or having versions out of sync.

This could be as simple as the flag we put for #RUST VER , or a place that actually manages the dependency. Regardless, we what this simple so it works across the board.

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