moby/buildkit dockerfile/1.4.0 on GitHub

# syntax=docker.io/docker/dockerfile:1.4.0

Note that docker/dockerfile:latest tag has not been updated yet and is available in staging at docker/dockerfile-upstream:latest.

COPY --link and ADD --link allow copying files with increased cache efficiency and rebase images without requiring them to be rebuilt. --link copies files to a separate layer and then uses new LLB MergeOp implementation to chain independent layers together. #2596 #2672 Documentation
Heredocs support have been promoted from labs channel to stable. This feature allows writing multiline inline scripts and files #2589 Documentation
Additional named build contexts can be passed to build to add or overwrite a stage or an image inside the build. A source for the context can be a local source, image, Git, or HTTP URL. #2521 #2550 #2549 #2693 Buildx Documentation
When using a cross-compilation stage, the target platform for a step is now seen on progress output #2576
BUILDKIT_SANDBOX_HOSTNAME build-arg can be used to set the default hostname for the RUN steps. #2373
Fixes for some cases where Heredocs incorrectly removed quotes from content #2442