Set up postgresql testcontainers

- add .json to .editorconfig
- add spring-configuration-metadata.json
- update README.md
- add testcontainers dependencies
- add PostgresContainer
- add PostgresInitializer

src/main/kotlin/ltd/hlaeja/test/container/PostgresInitializer.kt

@@ -0,0 +1,32 @@
+package ltd.hlaeja.test.container
+
+import org.springframework.boot.test.util.TestPropertyValues
+import org.springframework.context.ApplicationContextInitializer
+import org.springframework.context.ConfigurableApplicationContext
+import org.springframework.core.io.ClassPathResource
+import org.testcontainers.containers.PostgreSQLContainer
+
+@Suppress("unused")
+class PostgresInitializer : ApplicationContextInitializer<ConfigurableApplicationContext> {
+
+    override fun initialize(applicationContext: ConfigurableApplicationContext) {
+        postgres().apply {
+            TestPropertyValues.of(
+                "spring.r2dbc.url=r2dbc:pool:postgresql://$host:$firstMappedPort/$databaseName",
+                "spring.r2dbc.username=$username",
+                "spring.r2dbc.password=$password",
+            ).applyTo(applicationContext)
+        }
+    }
+
+    private fun postgres(): PostgreSQLContainer<*> = PostgreSQLContainer("postgres:17")
+        .withReuse(true)
+        .apply {
+            "postgres/schema.sql".let {
+                if (ClassPathResource(it).exists()) {
+                    withInitScript(it)
+                }
+            }
+            start()
+        }
+}