make postgres/gis verions in docker configurable
make postgres/gis verions in docker configurable

This adds some more buildtime args to allow you to pull postgres from a
different version and different image altogether. The defaults are fine
for current x86 builds, but for those building on ARM64 platforms,
they'll want to do add arguments to the build like:
  --build-arg POSTGRES=13 --build-arg POSTGIS=3.1
  --build-arg POSTGISIMAGE=ghcr.io/baosystems/postgis

Change-Id: Ied9f16ffdc0e63306fb7d102f5740f5865a1288c
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/281063
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Jacob Burroughs <8ecea6e385af5cf9f53123f5ca17fb5fd6a6d4b2@instructure.com>
QA-Review: Keith Garner <a1a80143a840c140f31c3495ebbd9ec906e09fed@instructure.com>
Product-Review: Keith Garner <a1a80143a840c140f31c3495ebbd9ec906e09fed@instructure.com>

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