Improve container test code

Replace `DockerImageNames` with a enum and relocate it from the
`testcontainers` to `container` package. The enum now also
becomes a common location that we can use to apply container
configuration such as timeouts.

Closes gh-41164

Co-authored-by: Phillip Webb <phil.webb@broadcom.com>

spring-boot-tests/spring-boot-integration-tests/spring-boot-loader-tests/src/intTest/java/org/springframework/boot/loader/LoaderIntegrationTests.java

@@ -33,7 +33,7 @@ import org.testcontainers.utility.DockerImageName;
 import org.testcontainers.utility.MountableFile;
 
 import org.springframework.boot.system.JavaVersion;
-import org.springframework.boot.testsupport.testcontainers.DisabledIfDockerUnavailable;
+import org.springframework.boot.testsupport.container.DisabledIfDockerUnavailable;
 import org.springframework.util.Assert;
 
 import static org.assertj.core.api.Assertions.assertThat;