Better error message if file/directory is ignored

[rumpl]

Description

With this .dockerignore file:

file

And this Dockerfile:

FROM scratch
COPY file .

When I try to build the image I get this error:

$ docker build -t test .
[+] Building 0.0s (4/4) FINISHED                                                                                                      docker:desktop-linux
 => [internal] load build definition from Dockerfile                                                                                                  0.0s
 => => transferring dockerfile: 62B                                                                                                                   0.0s
 => [internal] load .dockerignore                                                                                                                     0.0s
 => => transferring context: 45B                                                                                                                      0.0s
 => [internal] load build context                                                                                                                     0.0s
 => => transferring context: 2B                                                                                                                       0.0s
 => ERROR [1/1] COPY file .                                                                                                                           0.0s
------
 > [1/1] COPY file .:
------
Dockerfile:2
--------------------
   1 |     FROM scratch
   2 | >>> COPY file .
   3 |
--------------------
ERROR: failed to solve: failed to compute cache key: failed to calculate checksum of ref 30f4015d-0523-4ca8-b16b-d9d2abbc1430::aseapibm7ur3dq8lk8gjdig46: "/file": not found

It would be great is the error said something about the file being ingored by the .dockerignore file.

[crazy-max]

The following check https://docs.docker.com/reference/build-checks/copy-ignored-file/ should warn about the file being ignored. @tonistiigi Do you recall why we have this check behind experimental?

But I agree that we should add complementary info in build error if this file is actually being ignored like:

"/file": not found (ignored)

[crazy-max]

I see we set IncludePatterns and don't set ExcludePatterns during frontend convert:

buildkit/frontend/dockerfile/dockerfile2llb/convert.go

Lines 1580 to 1589 in 16fb975

	opts := append([]llb.CopyOption{&llb.CopyInfo{
	Mode: chopt,
	FollowSymlinks: true,
	CopyDirContentsOnly: true,
	IncludePatterns: patterns,
	AttemptUnpack: cfg.isAddCommand,
	CreateDestPath: true,
	AllowWildcard: true,
	AllowEmptyWildcard: true,
	}}, copyOpt...)

and so defer to LLB which will return not found. If we set ExcludePatterns we could then check on LLB side if the file is actually ignored and return a better error message.