BuildKit’s default output format is designed for human readability, but it can obscure the actual build process.
Here’s a look at BuildKit’s output in action, processing a simple Dockerfile.
# syntax=docker/dockerfile:1
FROM alpine:latest
RUN echo "Hello, world!" > /hello.txt
CMD ["cat", "/hello.txt"]
When you run docker build . --output type=docker,dest=output.tar, you get this output:
#0 building with BuildKit
#0 pulling image sha252c920298d (alpine:latest)
#0 pulling image sha252c920298d (alpine:latest) sha252c920298d
#0 sha252c920298d: Pulling from library/alpine
#0 Digest: sha252c920298d252c920298d252c920298d
#0 Status: Image is up to date
#0 RUN echo "Hello, world!" > /hello.txt
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 RUN echo "Hello, world!" > /hello.txt:
#0 [1/1] RUN echo "Hello, world!" > /hello.txt
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c920298d: digest: sha252c920298d size: 1189952
#0 sha252c92029