Spent a lot of time trying to figure out the right command to use, let's save it here once and for all :
curl -v \
-F "status=2" \
-F "notify=1" \
-F "ipa=@build/outputs/apk/app.apk" \
-H "X-HockeyAppToken: my-hockeyapp-token" \
https://rink.hockeyapp.net/api/2/apps/app_id/app_versions/upload