cache-from in buildkit returns invalid character error

Start by reproducing the two Docker builds with the shown Dockerfile, especially the second command using --cache-from=base:latest. Then trace BuildKit's cache-from handling after image metadata is loaded. Done means the invalid-character failure is explained and corrected or the expected behavior is documented with a regression test.

Hi!

I found strange behavior in this scenario.

I have python script (main.py):

print("Hello, World!")

Some requirements.txt:

python-dateutil==2.8.1

and Dockerfile:

# syntax=docker/dockerfile:1

FROM python:3-slim as base

RUN --mount=type=bind,target=./requirements.txt,src=./requirements.txt \
    --mount=type=cache,mode=0755,target=/root/.cache/pip \
    pip install -r requirements.txt

FROM base as main

COPY . /app
CMD ["python", "/app/main.py"]

I build first version with command and everything works fine:

docker build -t base --target base --build-arg BUILDKIT_INLINE_CACHE=1 .

After that I want to release a new version and reuse cache:

python-dateutil==2.8.2

docker build -t base --target base --build-arg BUILDKIT_INLINE_CACHE=1 --cache-from=base:latest .

And I get this output:

[+] Building 1.2s (5/5) FINISHED
 => [internal] load .dockerignore                                                                                                                                                                                                                                               0.0s
 => => transferring context: 2B                                                                                                                                                                                                                                                 0.0s 
 => [internal] load build definition from Dockerfile                                                                                                                                                                                                                            0.0s 
 => => transferring dockerfile: 340B                                                                                                                                                                                                                                            0.0s 
 => resolve image config for docker.io/docker/dockerfile:1                                                                                                                                                                                                                      0.5s 
 => CACHED docker-image://docker.io/docker/dockerfile:1@sha256:d2d74ff22a0e47b21f4bbde337e2ac4cd0a02a2226ef79264878db3dc7e87df8                                                                                                                                                 0.0s
 => => resolve docker.io/docker/dockerfile:1@sha256:d2d74ff22a0e47b21f4bbde337e2ac4cd0a02a2226ef79264878db3dc7e87df8                                                                                                                                                            0.0s 
 => [internal] load metadata for docker.io/library/python:3-slim                                                                                                                                                                                                                0.5s
invalid character 'c' looking for beginning of value

With --progress=plain it looks like this:

#5 [internal] load metadata for docker.io/library/python:3-slim
#5 sha256:1d53a8326d1210addccb4bc5ed043a5dc9dc3d4d221b85e1b898fd379832ee7b
#5 DONE 1.3s
invalid character 'c' looking for beginning of value

Is it correct behavior? Or I didn't understand BuildKit caching system?