Deploy java 11 app to GAE standard with gcloud CLI - google-app-engine

Question

I want to deploy a spring boot jar using the gcloud CLI. When I run gcloud app deploy it does not upload the jar file:
╔════════════════════════════════════════════════════════════╗
╠═ Uploading 0 files to Google Cloud Storage ═╣
╚════════════════════════════════════════════════════════════╝
File upload done.
When I run the gradle deploy command it also tells me that it uploaded one file. I assume that is the jar file that is missing.
#============================================================#
#= Uploading 1 file to Google Cloud Storage =#
#============================================================#
File upload done.
When I look at whats going on in cloud build, I see that it is fetching a manifest.json which looks like that:
{
"app.yaml": {
"sourceUrl": "https://storage.googleapis.com/staging.some- project.appspot.com/some-hash",
"sha1Sum": "some-hash"
},
"source-context.json": {
"sourceUrl": "https://storage.googleapis.com/staging.some-project.appspot.com/some-hash",
"sha1Sum": "some-hash"
}
}
When I see check how that manifest.json looks like when I deploy it with the gradle plugin, I see that it includes a reference to the jar file like:
"my-project-0.0.1-SNAPSHOT.jar": {
"sourceUrl": "https://storage.googleapis.com/staging.some-project.appspot.com/some-hash",
"sha1Sum": "some-hash"
}
What do I miss to configure?