[build] Incompatibility between end-of-command PATH and target relatively a Dockerfile with -f

Description

docker build allows to provide a PATH at the end of the command to locate the Dockerfile there, or to target a Dockerfile with - f.

Reading at the documentation/manpage, it leads to think PATH becomes the working directory, and as such if we combine both we could use -f to target the Dockerfile relatively to PATH.

Reproduce

Picture this structure:

repo_root
|___subdirectory
       |____Dockerfile

One would expect here that we could be at repo_root and run docker build -f Dockerfile subdirectory as Dockerfile is at current location (.) when starting from subdirectory.

That's what lead to think the manpage.

The result however is this: ERROR: failed to build: failed to solve: failed to read dockerfile: open Dockerfile: no such file or directory

Note that although the error message is different, the issue exist for both legacy builder and BuildKit.

Expected behavior

The example given above should work, or if it's expected that it doesn't, the manpage might need a little rewording.

docker version

What's provided in GitHub Actions context first, then reproduced on an ArchLinux computer I don't have at hand to paste the block. Docker 29.8.0 on Arch (https://archlinux.org/packages/extra/x86_64/docker/) reproduced with legacy builder and BuildKit one (https://archlinux.org/packages/extra/x86_64/docker-buildx/)

docker info

Same as above, don't need to have the build running anyway.

Additional Info

No response

Start by reproducing docker build -f Dockerfile subdirectory with both the legacy builder and BuildKit, then inspect the docker build documentation and manpage wording about PATH and Dockerfile location. Done means either the command works as described or the documentation clearly explains the limitation and correct usage.