[WebApi] Add Integration tests to Postgres DB via Test Containers

[gclodge]

Given that we're using PostgreSQL & PostGIS under the hood (via Npgsql & EF7) we aren't able to swap to a simple in-memory SQLite or similar DB or integration testing.

As such, I'd like to implement some proper integration tests for the WebApi that use 'Test Containers' to actually interact with a fully configured DB following Matthias Guntert's excellent walkthrough here:

• https://www.azureblue.io/asp-net-core-integration-tests-with-test-containers-and-postgres/