docker + ktor + jib -- JDK-21 and how to sort out port setting.

build.gradle.kts

ktor {
    docker {
        jreVersion.set(JavaVersion.VERSION_21)
        portMappings.set(listOf(
            io.ktor.plugin.features.DockerPortMapping(
                15551, 15551,
                io.ktor.plugin.features.DockerPortMappingProtocol.TCP
            )
        ))
    }
}

jib {
    from {
        image = "eclipse-temurin:21-jdk-alpine"
    }
}

This is, if your app wants to listen to port 15551. Docker/ktor by default gives 8080 if not told otherwise.