postgresql-15-postgis-3 not found

When running make test, it fails to execute because postgresql-15-postgis-3 isn't found.  I had to change this to postgresql-17-postgis-3 in Dockerfile.tests.

To reproduce, 

`
git clone https://github.com/stac-utils/stac-fastapi-pgstac
cd stac-fastapi-pgstac
python -m venv .venv
. .venv/bin/activate
python -m pip install -e ".[dev,server,docs]"
make test
`

