changed image to openjdk:15, use gradlew

1 job for feature/ci-with-gradle-wrapper in 4 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
failed #40890
build

00:00:04

 
Name Stage Failure
failed
build Test
Fetching changes with git depth set to 50...
Reinitialized existing Git repository in /builds/oesa/ecs-tutorial/.git/
Checking out 3b696d04 as feature/ci-with-gradle-wrapper...

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