Why might a Spring Boot app work fine locally (run from the IDE) but fail to connect to Postgres once containerized?

Running directly on a developer machine, 'localhost' correctly refers to wherever Postgres happens to be running (often a local install or a port-forwarded container). Once the Spring Boot app itself is containerized, 'localhost' refers to the app's own container, not the database's — the connection string must use the database's service/container name (e.g., jdbc:postgresql://db:5432/appdb) so Docker's embedded DNS resolves it correctly.