Ah, well, the output of what I wrote above isn't actually a Docker image. It ends up writing a cross-compiled binary to the local FS, which I then later attach

Ah, well, the output of what I wrote above isn't actually a Docker image. It ends up writing a cross-compiled binary to the local FS, which I then later attach to a GitHub Release. (The goal was to automatically attach platform binaries to a release once it was made.) ...unless your Dockerfile ends up doing that too? If so I'd be interested to see it if you're willing to share!