Building for ppc64le

I'm trying to build the deb package for Filebeat for ppc64le . I've followed the developer guide and setup a Go environment with Python 3.7, Mage, and Virtualenv on my system. When I navigate to beats/filebeat directory and run PLATFORMS=linux/ppc64le make release it says:

2020/03/26 13:41:58 Found Elastic Beats dir at /luke-perm/go/src/github.com/elastic/beats
Generated fields.yml for filebeat to /luke-perm/go/src/github.com/elastic/beats/filebeat/fields.yml
No fields files for module apache2
Generated fields.yml for filebeat to /luke-perm/go/src/github.com/elastic/beats/filebeat/fields.yml
>> Building filebeat.yml for linux/amd64
>> Building filebeat.reference.yml for linux/amd64
>> Building filebeat.docker.yml for linux/amd64
2020/03/26 13:42:03 Found Elastic Beats dir at /luke-perm/go/src/github.com/elastic/beats
Generated fields.yml for filebeat to /luke-perm/go/src/github.com/elastic/beats/filebeat/build/fields/fields.all.yml
>> package: Skipping because the platform list is empty
>> Testing package contents
package ran for 10.335682943s

It looks like it is not recognizing the value of PLATFORMS variable...

I also tried using the Docker cross-build container from https://github.com/elastic/golang-crossbuild to build for ppc64le on an x86 machine, but when I run:

docker run -it --rm \
  -v $GOPATH/src/github.com/elastic/beats:/go/src/github.com/elastic/beats \
  -w /go/src/github.com/elastic/beats/filebeat \
  -e CGO_ENABLED=1 \
  docker.elastic.co/beats-dev/golang-crossbuild:1.13.9-ppc \
  --build-cmd "make release" \
  -p "linux/ppc64le"

it complains about mage command not existing...

Can anyone tell me what I'm doing wrong? Thanks.