changed image to openjdk:15, use gradlew

1 job for feature/ci-with-gradle-wrapper in 19 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
failed #40888
build

00:00:19

 
Name Stage Failure
failed
build Test
Skipping Git submodules setup
Executing "step_script" stage of the job script
$ wget -O- https://github.com/mlange-42/yarner/releases/download/${YARNER_VERSION}/yarner-${YARNER_VERSION}-linux.tar.gz | tar xz
/usr/bin/bash: line 97: wget: command not found

gzip: stdin: unexpected end of file
tar: Child returned status 1
tar: Error is not recoverable: exiting now
ERROR: Job failed: exit code 1