linter: do not attempt to check for copying ignored file when negated patterns exist

frontend/dockerfile/dockerfile2llb/convert.go

@@ -2041,7 +2041,13 @@ func addReachableStages(s *dispatchState, stages map[*dispatchState]struct{}) {
 }
 
 func validateCopySourcePath(src string, cfg *copyConfig) error {
-	if cfg.ignoreMatcher == nil {
+	// Do not validate copy source paths if there is no dockerignore file
+	// or if the dockerignore file contains exclusions.
+	//
+	// Exclusions are too difficult to statically determine if they're proper
+	// because it's ok for a directory to be excluded and a file inside the directory
+	// to be negated.
+	if cfg.ignoreMatcher == nil || cfg.ignoreMatcher.Exclusions() {
 		return nil
 	}
 	cmd := "Copy"

frontend/dockerfile/dockerfile_lint_test.go

@@ -230,6 +230,16 @@ COPY ./Dockerfile .
 		Dockerfile:   dockerfile,
 		DockerIgnore: dockerignore,
 	})
+
+	// No warnings should occur if we copy the directory.
+	dockerfile = []byte(`
+FROM scratch
+COPY . .
+	`)
+	checkLinterWarnings(t, sb, &lintTestParams{
+		Dockerfile:   dockerfile,
+		DockerIgnore: dockerignore,
+	})
 }
 
 func testSecretsUsedInArgOrEnv(t *testing.T, sb integration.Sandbox) {