Docker/Gradle/Kotlin/SpringBoot problem: doesn't cache gradle plugins

build.gradle.kts

// Created with https://start.spring.io, select gradle / kotlin
import org.jetbrains.kotlin.gradle.tasks.KotlinCompile

/* Try 3: use old gradle structure
buildscript {
  repositories {
      jcenter()
    maven {
      url = uri("https://plugins.gradle.org/m2/")
    }
  }
  dependencies {
    classpath ("org.springframework.boot:spring-boot-gradle-plugin:2.2.4.RELEASE")
    classpath ("io.spring.gradle:dependency-management-plugin:1.0.9.RELEASE")
    classpath ("org.jetbrains.kotlin:kotlin-gradle-plugin:1.3.61")
    classpath ("org.jetbrains.kotlin:kotlin-allopen:1.3.61")
  }
}

apply (plugin = "kotlin")
apply (plugin = "kotlin-spring")
apply(plugin = "org.springframework.boot")            // https://plugins.gradle.org/plugin/org.springframework.boot
apply(plugin = "io.spring.dependency-management")     // https://plugins.gradle.org/plugin/io.spring.dependency-management
//apply(plugin = "org.jetbrains.kotlin.jvm")            // https://plugins.gradle.org/plugin/org.jetbrains.kotlin.jvm
//apply(plugin = "org.jetbrains.kotlin.plugin.spring")  // https://plugins.gradle.org/plugin/org.jetbrains.kotlin.plugin.spring
*/

 plugins {
    id("org.springframework.boot") version "2.2.4.RELEASE"
    id("io.spring.dependency-management") version "1.0.9.RELEASE"
    kotlin("jvm") version "1.3.61"
    kotlin("plugin.spring") version "1.3.61"
}

group = "adamatti"
version = "0.0.1-SNAPSHOT"
java.sourceCompatibility = JavaVersion.VERSION_1_8

repositories {
    mavenCentral()
}

dependencies {
    implementation("org.springframework.boot:spring-boot-starter")
    implementation("org.jetbrains.kotlin:kotlin-reflect")
    implementation("org.jetbrains.kotlin:kotlin-stdlib-jdk8")
    testImplementation("org.springframework.boot:spring-boot-starter-test") {
        exclude(group = "org.junit.vintage", module = "junit-vintage-engine")
    }
}

tasks.withType<Test> {
    useJUnitPlatform()
}

tasks.withType<KotlinCompile> {
    kotlinOptions {
        freeCompilerArgs = listOf("-Xjsr305=strict")
        jvmTarget = "1.8"
    }
}

Dockerfile

FROM gradle:6.1.1-jdk8 as builder

USER root

################################################################################ Try 1: have kotlin. Didn't work
#ENV KOTLIN_VERSION=1.3.61 \
#    KOTLIN_HOME=/usr/local/kotlin

#RUN cd  /tmp && \
#    wget -k "https://github.com/JetBrains/kotlin/releases/download/v${KOTLIN_VERSION}/kotlin-compiler-${KOTLIN_VERSION}.zip"  && \
#    unzip "kotlin-compiler-${KOTLIN_VERSION}.zip" && \
#    mkdir -p "${KOTLIN_HOME}" && \
#    mv "/tmp/kotlinc/bin" "/tmp/kotlinc/lib" "${KOTLIN_HOME}" && \
#    rm ${KOTLIN_HOME}/bin/*.bat && \
#    chmod +x ${KOTLIN_HOME}/bin/* && \
#    ln -s "${KOTLIN_HOME}/bin/"* "/usr/bin/" && \
#    rm -rf /tmp/* /var/cache/apk/*
################################################################################

RUN mkdir /app
WORKDIR /app

################################################################################ Try 2: persist gradle/kotlin temp files. Didn't work
#RUN mkdir /gradle-tmp
#ENV JAVA_OPTS=-Djava.io.tmpdir=/gradle-tmp
################################################################################

COPY build.gradle.kts /app
COPY settings.gradle.kts /app
RUN gradle dependencies --build-cache --no-daemon

COPY src /app/src
RUN gradle build --no-daemon -x test

# Goal: use '--offline'. Doesn't work, gradle still try download plugins
RUN gradle build --no-daemon --offline -x test 

Main.kt

// src/main/kotlin
fun main(args: Array<String>){
    println("Hello world")
}

settings.gradle.kts

rootProject.name = "sample"