Fix behavior of absolute paths in .dockerignore

According to documentation (https://docs.docker.com/engine/reference/builder/#dockerignore-file), absolute paths like `/foo/bar` should have the same effect as `foo/bar`. This is not the case today.

This fix normalize paths when reading the .dockerignore file by removing
leading slashes.

Signed-off-by: Simon Ferquel <simon.ferquel@docker.com>

builder/dockerignore/dockerignore.go

@@ -38,8 +38,23 @@ func ReadAll(reader io.Reader) ([]string, error) {
 		if pattern == "" {
 			continue
 		}
-		pattern = filepath.Clean(pattern)
-		pattern = filepath.ToSlash(pattern)
+		// normalize absolute paths to paths relative to the context
+		// (taking care of '!' prefix)
+		invert := pattern[0] == '!'
+		if invert {
+			pattern = strings.TrimSpace(pattern[1:])
+		}
+		if len(pattern) > 0 {
+			pattern = filepath.Clean(pattern)
+			pattern = filepath.ToSlash(pattern)
+			if len(pattern) > 1 && pattern[0] == '/' {
+				pattern = pattern[1:]
+			}
+		}
+		if invert {
+			pattern = "!" + pattern
+		}
+
 		excludes = append(excludes, pattern)
 	}
 	if err := scanner.Err(); err != nil {

builder/dockerignore/dockerignore_test.go

@@ -25,7 +25,7 @@ func TestReadAll(t *testing.T) {
 	}
 
 	diName := filepath.Join(tmpDir, ".dockerignore")
-	content := fmt.Sprintf("test1\n/test2\n/a/file/here\n\nlastfile")
+	content := fmt.Sprintf("test1\n/test2\n/a/file/here\n\nlastfile\n# this is a comment\n! /inverted/abs/path\n!\n! \n")
 	err = ioutil.WriteFile(diName, []byte(content), 0777)
 	if err != nil {
 		t.Fatal(err)
@@ -42,16 +42,28 @@ func TestReadAll(t *testing.T) {
 		t.Fatal(err)
 	}
 
+	if len(di) != 7 {
+		t.Fatalf("Expected 5 entries, got %v", len(di))
+	}
 	if di[0] != "test1" {
 		t.Fatal("First element is not test1")
 	}
-	if di[1] != "/test2" {
-		t.Fatal("Second element is not /test2")
+	if di[1] != "test2" { // according to https://docs.docker.com/engine/reference/builder/#dockerignore-file, /foo/bar should be treated as foo/bar
+		t.Fatal("Second element is not test2")
 	}
-	if di[2] != "/a/file/here" {
-		t.Fatal("Third element is not /a/file/here")
+	if di[2] != "a/file/here" { // according to https://docs.docker.com/engine/reference/builder/#dockerignore-file, /foo/bar should be treated as foo/bar
+		t.Fatal("Third element is not a/file/here")
 	}
 	if di[3] != "lastfile" {
 		t.Fatal("Fourth element is not lastfile")
 	}
+	if di[4] != "!inverted/abs/path" {
+		t.Fatal("Fifth element is not !inverted/abs/path")
+	}
+	if di[5] != "!" {
+		t.Fatalf("Sixth element is not !, but %s", di[5])
+	}
+	if di[6] != "!" {
+		t.Fatalf("Sixth element is not !, but %s", di[6])
+	}
 }