Build output always shows a mysterious .dockerignore

This is one of those things that have probably always been there, but I just now noticed when posting a comment on https://github.com/moby/buildkit/issues/2511#issuecomment-1431412029.

When building a Dockerfile, which could be from stdin;

docker build -t foo -<<'EOF'
FROM alpine
RUN echo hello
EOF

Building the above shows:

[+] Building 1.9s (6/6) FINISHED
=> [internal] load build definition from Dockerfile                                                                    0.1s
=> => transferring dockerfile: 64B                                                                                     0.0s
=> [internal] load .dockerignore                                                                                       0.0s
=> => transferring context: 2B                                                                                         0.0s
=> [internal] load metadata for docker.io/library/alpine:latest                                                        0.9s
=> [1/2] FROM docker.io/library/alpine@sha256:69665d02cb32192e52e07644d76bc6f25abeb5410edc1c7a81a10ba3f0efb90a         0.5s
=> => resolve docker.io/library/alpine@sha256:69665d02cb32192e52e07644d76bc6f25abeb5410edc1c7a81a10ba3f0efb90a         0.0s
=> => sha256:63b65145d645c1250c391b2d16ebe53b3747c295ca8ba2fcb6b0cf064a4dc21c 3.37MB / 3.37MB                          0.2s
=> => sha256:69665d02cb32192e52e07644d76bc6f25abeb5410edc1c7a81a10ba3f0efb90a 1.64kB / 1.64kB                          0.0s
=> => sha256:e2e16842c9b54d985bf1ef9242a313f36b856181f188de21313820e177002501 528B / 528B                              0.0s
=> => sha256:b2aa39c304c27b96c1fef0c06bee651ac9241d49c4fe34381cab8453f9a89c7d 1.47kB / 1.47kB                          0.0s
=> => extracting sha256:63b65145d645c1250c391b2d16ebe53b3747c295ca8ba2fcb6b0cf064a4dc21c                               0.2s
=> [2/2] RUN echo hello                                                                                                0.4s
=> exporting to image                                                                                                  0.1s
=> => exporting layers                                                                                                 0.1s
=> => writing image sha256:136f47c90fc5b4fb4f1a440f668c971b936deeb060b6f833fe13e4fb3ee961c4                            0.0s
=> => naming to docker.io/library/foo

Notice that the output always shows that it's loading a .dockerignore, even though this build does not have a .dockerignore.

=> [internal] load .dockerignore                                                                                       0.0s
=> => transferring context: 2B                                                                                         0.0s

I'm guessing this is some implementation detail (perhaps an empty {} struct is sent accross the wire), but this is confusing, as a user could come to the conclusion that a .dockerignore is present "somewhere".

I wonder if we can either;

• Suppress that output altogether (if theres no .dockerignore)
• Suppress the 2B size
• Or change the output to something informative (i.e., it checked if a .dockerignore is present, and found that there isn't)

Reproduce the reported behavior with the Dockerfile-from-stdin command and compare the emitted .dockerignore progress lines. Trace the BuildKit output path that handles the absent ignore file; done means builds without a .dockerignore no longer display a misleading file load or size, with coverage for this case.