populate-depot: Have an overall version number for the depot as a whole
With some changes coming in via pressure-vessel, some via the container image and some via the scripts, any or all of which might be updated in any given container build, it's getting a bit confusing. If we assign an overall version number to the whole container, we can talk about a particular depot build unambiguously.
There is a corresponding change in our internal CI machinery to make this effective, but that's in non-public code.