failed to solve: failed to compute cache key: failed to calculate checksum of ref

I often have builds failing with this error:

failed to solve: failed to compute cache key: failed to calculate checksum of ref pzhz5wxhfuk4nt6gvf90y7xb3::uydm9wxbcjq4otmpf0mm502cn: "/docker-entrypoint.sh": not found

Whereas /docker-entrypoint.sh is just an example. The file exists in the path ./docker/docker-entrypoint.sh and obviously the COPY command of Dockerfile does not even fail?!

After fiddling around for sometimes hours, pruning buildkit-cache and so on, suddenly the build works and I don't even understand what was wrong.

In most cases using pure docker for build even works (without buildkit features), when docker-compose build with buildkit enabled fails!

I would like to understand if this is a bug or just an user unfriendly error-message that leads to wrong understanding the main problem?!

In some cases I think it worked to move the file to be copied in a subfolder like:
./docker/rootfs//docker-entrypoint.boot.sh and modifying COPY to COPY --chmod=a+x ./rootfs/docker-entrypoint.boot.sh /docker-entrypoint.boot.sh

But why doesn't it then work for the path ./docker/docker-entrypoint.boot.sh?!

The Dockerfile-part responsible looks like this f.e.:

COPY --chmod=a+x ./docker-entrypoint.boot.sh /docker-entrypoint.boot.sh

docker-compose.yml (minimal) example:

version: "3.7"

services:
  app-build:
    image: my-app:local
    build:
      context: ./docker
      dockerfile: Dockerfile

In most cases I'm using docker-compose.yml which is in project-root and the docker files are in docker sub-directory:

docker/
  docker-entrypoint.boot.sh
  Dockerfile
docker-compose.yml

Start with the minimal Dockerfile and docker-compose.yml example in the issue, using the shown docker/ directory layout. Reproduce the checksum error with BuildKit enabled, then compare it with a non-BuildKit build and the alternate subfolder path. Done means identifying whether the failure is reproducible and whether the error reflects a BuildKit bug or misleading diagnostics.