replace dockerfile/dockerignore with patternmatcher/ignorefile

The BuildKit dockerignore package was migrated to the patternmatcher
repository / module. This patch updates our uses of the BuildKit package
with its new location.

A small local change was made to keep the format of the existing error message,
because the "ignorefile" package is slightly more agnostic in that respect
and doesn't include ".dockerignore" in the error message.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/image/build/dockerignore.go

@@ -1,11 +1,12 @@
 package build
 
 import (
+	"fmt"
 	"os"
 	"path/filepath"
 
-	"github.com/moby/buildkit/frontend/dockerfile/dockerignore"
 	"github.com/moby/patternmatcher"
+	"github.com/moby/patternmatcher/ignorefile"
 )
 
 // ReadDockerignore reads the .dockerignore file in the context directory and
@@ -22,7 +23,11 @@ func ReadDockerignore(contextDir string) ([]string, error) {
 	}
 	defer f.Close()
 
-	return dockerignore.ReadAll(f)
+	patterns, err := ignorefile.ReadAll(f)
+	if err != nil {
+		return nil, fmt.Errorf("error reading .dockerignore: %w", err)
+	}
+	return patterns, nil
 }
 
 // TrimBuildFilesFromExcludes removes the named Dockerfile and .dockerignore from