How to configure 'mainClassName' for Spring Boot Application in a Kotlin Gradle multi-module project? - spring-boot

Question

I have a Gradle multi-module project in Kotlin using Spring Boot in some of the modules.
I use the 'org.springframework.boot' and 'io.spring.dependency-management' plugin in the project's root build.gradle which besides other things provide the bootRun task.
AFAIK I need to configure the mainClassName (or entry point) like this:
springBoot {
mainClassName = 'de.shinythings.hexagon.HexagonApplicationKt'
}
While this works in the subproject that contains the actual Spring Boot application it does not work in the root project.
I want to be able to configure it in the root project because I want to be able to run my project with ./gradlew bootRun instead of ./gradlew configuration:bootRun.
Other tasks like ./gradlew build or ./gradlew test work fine.
Here is the root project's build.gradle (link):
plugins {
id 'org.jetbrains.kotlin.jvm' version '1.4.10'
id 'org.springframework.boot' version '2.3.4.RELEASE'
id 'io.spring.dependency-management' version '1.0.10.RELEASE'
}
repositories {
mavenCentral()
}
dependencies {
implementation project(':configuration')
}
springBoot {
mainClassName = 'de.shinythings.hexagon.HexagonApplicationKt'
}
subprojects {
group = 'de.shinythings.hexagon'
version = '0.0.1-SNAPSHOT'
repositories {
mavenCentral()
}
ext {
mockKVersion = '1.10.2'
}
apply plugin: 'org.jetbrains.kotlin.jvm'
apply plugin: 'io.spring.dependency-management'
dependencies {
implementation("org.jetbrains.kotlin:kotlin-reflect")
implementation("org.jetbrains.kotlin:kotlin-stdlib-jdk8")
}
dependencyManagement {
imports {
mavenBom('org.springframework.boot:spring-boot-dependencies:2.3.4.RELEASE')
}
}
compileKotlin {
kotlinOptions {
jvmTarget = JavaVersion.VERSION_1_8
freeCompilerArgs = ["-Xjsr305=strict"]
allWarningsAsErrors = true
}
}
compileTestKotlin {
kotlinOptions {
jvmTarget = JavaVersion.VERSION_1_8
freeCompilerArgs = ["-Xjsr305=strict"]
allWarningsAsErrors = true
}
}
}
In the subproject/module this setting works (link) :
plugins {
id 'org.springframework.boot'
id 'org.jetbrains.kotlin.plugin.spring' version '1.4.10'
}
dependencies {
implementation project(':common')
implementation project(':application')
implementation project(':adapters:persistence')
implementation project(':adapters:web')
implementation 'org.springframework.boot:spring-boot-starter'
implementation('com.h2database:h2')
}
springBoot {
mainClassName = 'de.shinythings.hexagon.HexagonApplicationKt'
}
What do I have to do to configure the mainClassName is the root's project build.gradle?