changed image to openjdk:15, use gradlew

1 job for feature/ci-with-gradle-wrapper in 18 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
failed #40813
build

00:00:18

 
Name Stage Failure
failed
build Test
Initialized empty Git repository in /builds/oesa/ecs-tutorial/.git/
Created fresh repository.
Checking out d7f8715c as feature/ci-with-gradle-wrapper...

Skipping Git submodules setup
Executing "step_script" stage of the job script
$ apt-get install wget
/usr/bin/bash: line 97: apt-get: command not found
ERROR: Job failed: exit code 1