How can I deploy a Spring MVC Gradle app to Heroku?

I have a spring MVC application and I would like to deploy it to Heroku. I keep getting this no main manifest attribute in .jar every time. Spring MVC doesn't have a main method that I can specify to manifest. How could I fix this issue and deploy it? I am running out of ideas.

Error from Heroku:

2022-01-15T11:41:11.336214+00:00 heroku[web.1]: Starting process with command `java $JAVA_OPTS -Dserver.port=47818 -jar app.jar`
2022-01-15T11:41:12.129299+00:00 app[web.1]: Setting JAVA_TOOL_OPTIONS defaults based on dyno size. Custom settings will override them.
2022-01-15T11:41:12.133003+00:00 app[web.1]: Picked up JAVA_TOOL_OPTIONS: -Xmx300m -Xss512k -XX:CICompilerCount=2 -Dfile.encoding=UTF-8
2022-01-15T11:41:12.219291+00:00 app[web.1]: no main manifest attribute, in app.jar
2022-01-15T11:41:12.335236+00:00 heroku[web.1]: Process exited with status 1
2022-01-15T11:41:12.459762+00:00 heroku[web.1]: State changed from starting to crashed
2022-01-15T11:41:17.000000+00:00 app[api]: Build succeeded

Procfile contents:

web: java $JAVA_OPTS -Dserver.port=$PORT -jar app.jar

build.gradle contents:

plugins {
    id 'java'
    id 'org.gretty' version '3.0.6'
}


group 'org.example'
version '1.2'

gretty {
    contextPath = '/'
}

jar {
    manifest {
        attributes("Implementation-Title": "Gradle",
                "Implementation-Version": archiveVersion
        )
    }
}

repositories {
    mavenCentral()
}

def springVersion = "5.3.10"

dependencies {

    implementation('com.github.jsimone:webapp-runner:8.5.11.3')
    compileOnly group: 'javax.servlet', name: 'javax.servlet-api', version: '4.0.1'

    implementation group: 'com.fasterxml.jackson.core',
            name: 'jackson-databind',
            version: '2.12.5'

    implementation group: 'org.hibernate',
            name: 'hibernate-validator',
            version: '6.2.0.Final'

    implementation group: 'org.hibernate',
            name: 'hibernate-core',
            version: '5.5.8.Final'

    implementation group: 'org.springframework',
            name: 'spring-orm',
            version: springVersion

    implementation group: 'org.postgresql',
            name: 'postgresql',
            version: '42.2.23'

    implementation group: 'org.springframework',
            name: 'spring-jdbc',
            version: springVersion

    implementation group: 'org.springframework',
            name: 'spring-webmvc',
            version: springVersion

    implementation group: 'org.springframework',
            name: 'spring-context',
            version: springVersion

    compileOnly group: 'org.projectlombok',
            name: 'lombok',
            version: '1.18.20'

    annotationProcessor group: 'org.projectlombok',
            name: 'lombok',
            version: '1.18.20'

}

test {
    useJUnitPlatform()
}

task stage(type: Copy, dependsOn: [clean, build]) {
    from jar.archivePath
    into project.rootDir
    rename {
        'app.jar'
    }
}
stage.mustRunAfter(clean)