Unfortunately this syntax is not generally supported yet - it's only supported with the buildkit backend and only landed in the 1.3 "labs" release. It was moved to stable in early 2022 (see https://github.com/moby/buildkit/issues/2574), so that seems to be better, but I think may still require a syntax directive to enable.

Many other dockerfile build tools still don't support it, e.g. buildah (see https://github.com/containers/buildah/issues/3474)

Useful now if you have control over the environment your images are being built in, but I'm excited to the future where it's commonplace!

And AFAIK buildkit is still a real pain to troubleshoot, as you can't use intermediate stages :/

You can put stuff in a script file and just run that script too.

You can! It's experimental but https://github.com/docker/buildx/pull/1168

Tracking for this: https://github.com/docker/buildx/issues/1104

excellent, thanks!

this has been a serious pain in my side for a while, both for my own debugging and for telling people I try to help "you're gonna have to start over and do X or this will take hours longer".