feat: Use Testcontainers for controller tests

[thorlauridsen]

Testcontainers allow us to spin up a temporary Docker image during tests. For this project, we will spin up a temporary PostgreSQL database to run the controller tests against a real PostgreSQL database instead of an in-memory H2 database. There are differences between PostgreSQL and H2 databases so if you plan to use a PostgreSQL database in production, it also makes sense to mimic this by using a PostgreSQL database in the tests. H2 databases are useful for quickly testing and running the system locally, but it is better to use Testcontainers to run a real PostgreSQL database for the controller tests.

Use Testcontainers for controller tests:

• Add Testcontainers annotation in controller tests.
• Define a PostgreSQLContainer which will be used for the tests.
• Add application-postgres.yml with connection settings for PostgreSQL.
• Add ActiveProfiles("postgres") annotation in controller test which ensures it uses application-postgres.yml.
• Update README.md.