What's happening here is that you're misusing the @ flag for the --data-binary option. The man page for curl describes its use quite clearly
If you start the data with the letter @, the rest should be a file name to read the data from, or - if you want curl to read the data from stdin.
What you wrote instructed curl to use the file called app-debug.apk from the subdirectory example.com:
curl ... --data-binary @example.com/app-debug.apk
It then told you that it couldn't find that file, but then proceeded to complete the rest of the command, resulting in an empty POST:
Warning: Couldn't read data from file
Warning: "https://s3-eu-west-1.amazonaws.com/files.greenhouseci.com/projects/6d
Warning: 7f406a-2a65-4be0-83ee-75ca0afae7c9/builds/24f78eb5-819f-44d5-9c21-edce
Warning: 4dc9f253/artefacts/app-debug.apk", this makes an empty POST.
Nowhere does it say that @ can be used to reference a URI, so what you need to do is to get the artifact yourself and then POST it to your server. Something like either of these suggestions could work
Download the POST content and then upload it
curl https://example.com/app-debug.apk >app-debug.apk
curl -X POST https://xxx.example.com:443/api/storage/upload -H "Content-Type:application/octet-stream" --data-binary @app-debug.apk
Stream the POST content and upload it
curl https://example.com/app-debug.apk |
curl -X POST https://xxx.example.com:443/api/storage/upload -H "Content-Type:application/octet-stream" --data-binary @-
