changed image to openjdk:15, use gradlew

1 job for feature/ci-with-gradle-wrapper in 5 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
failed #40910
build

00:00:05

 
Name Stage Failure
failed
build Test
  --> Writing file code/src/main/java/grassing/util/MathUtil.java
Copying code file gradle/gradle/wrapper/gradle-wrapper.jar to gradle/wrapper/gradle-wrapper.jar
Copying code file gradle/gradle/wrapper/gradle-wrapper.properties to gradle/wrapper/gradle-wrapper.properties
Copying code file gradle/gradlew to gradlew
Copying code file gradle/gradlew.bat to gradlew.bat
$ cd code
$ ./gradlew build
/usr/bin/bash: line 105: ./gradlew: Permission denied
ERROR: Job failed: exit code 1