I tried using both JSON and p12 flavours but both result in the following failure:

[workspace] $ docker tag --force=true 2815f91921d7 eu.gcr.io/project-id/reporting-server:latest
[workspace] $ docker inspect 2815f91921d7
[workspace] $ docker push eu.gcr.io/project-id/reporting-server:12
The push refers to a repository [eu.gcr.io/project-id/reporting-server] (len: 1)
2815f91921d7: Preparing
Post https://eu.gcr.io/v2/project-id/reporting-server/blobs/uploads/: token auth attempt for registry: https://eu.gcr.io/v2/token?account=_token&scope=repository%3Aproject-id%2Freporting-server%3Apush%2Cpull&service=eu.gcr.io request failed with status: 403 Forbidden
Build step 'Docker Build and Publish' marked build as failure

Sadly it is not clear precisely what has failed.

The user that created the token successfully pushed the image manually using the gcloud tool so there was no permission problem at that level.