fix: dot path normalized correctly for COPY

In my previous fix on #4825, I had removed this line
knowing that all that had been addressed in `pahtRelativeToWorkingDir`:

```go
	if cfg.params.DestPath == "." // <-- this one
		|| cfg.params.DestPath == "" ||
		cfg.params.DestPath[len(cfg.params.DestPath)-1] == filepath.Separator {
		dest += string(filepath.Separator)
	}
```

However, I had overlooked the `"."` and `""` scenarios. `""`.
The  `"/"`case is handled correctly in `system.NormalizePath()`.

This change therefore undoes this, to make sure "." is transformed
correctly to "./" during COPY operation, same to "" -> "./". This
is important especially for WORKDIR that are not `/`, so that
`COPY --link` operations are handled properly.

fixes #5070

Signed-off-by: Anthony Nandaa <[email protected]>

frontend/dockerfile/dockerfile2llb/convert.go

@@ -1763,9 +1763,6 @@ func pathRelativeToWorkingDir(s llb.State, p string, platform ocispecs.Platform)
 		return "", err
 	}
 
-	if len(p) == 0 {
-		return dir, nil
-	}
 	p, err = system.CheckSystemDriveAndRemoveDriveLetter(p, platform.OS)
 	if err != nil {
 		return "", errors.Wrap(err, "removing drive letter")
@@ -1774,6 +1771,12 @@ func pathRelativeToWorkingDir(s llb.State, p string, platform ocispecs.Platform)
 	if system.IsAbs(p, platform.OS) {
 		return system.NormalizePath("/", p, platform.OS, true)
 	}
+
+	// add slashes for "" and "." paths
+	// "" is treated as current directory and not necessariy root
+	if p == "." || p == "" {
+		p = "./"
+	}
 	return system.NormalizePath(dir, p, platform.OS, true)
 }
 

frontend/dockerfile/dockerfile_test.go

@@ -142,6 +142,8 @@ var allTests = integration.TestFuncs(
 	testNamedMultiplatformInputContext,
 	testNamedFilteredContext,
 	testEmptyDestDir,
+	testCopyLinkDotDestDir,
+	testCopyLinkEmptyDestDir,
 	testCopyChownCreateDest,
 	testCopyThroughSymlinkContext,
 	testCopyThroughSymlinkMultiStage,
@@ -447,6 +449,67 @@ RUN [ "$(cat testfile)" == "contents0" ]
 	require.NoError(t, err)
 }
 
+func testCopyLinkDotDestDir(t *testing.T, sb integration.Sandbox) {
+	integration.SkipOnPlatform(t, "windows")
+	f := getFrontend(t, sb)
+
+	dockerfile := []byte(`
+FROM busybox
+WORKDIR /var/www
+COPY --link testfile .
+RUN [ "$(cat testfile)" == "contents0" ]
+`)
+
+	dir := integration.Tmpdir(
+		t,
+		fstest.CreateFile("Dockerfile", dockerfile, 0600),
+		fstest.CreateFile("testfile", []byte("contents0"), 0600),
+	)
+
+	c, err := client.New(sb.Context(), sb.Address())
+	require.NoError(t, err)
+	defer c.Close()
+
+	_, err = f.Solve(sb.Context(), c, client.SolveOpt{
+		LocalMounts: map[string]fsutil.FS{
+			dockerui.DefaultLocalNameDockerfile: dir,
+			dockerui.DefaultLocalNameContext:    dir,
+		},
+	}, nil)
+	require.NoError(t, err)
+}
+
+func testCopyLinkEmptyDestDir(t *testing.T, sb integration.Sandbox) {
+	integration.SkipOnPlatform(t, "windows")
+	f := getFrontend(t, sb)
+
+	dockerfile := []byte(`
+FROM busybox
+WORKDIR /var/www
+ENV empty=""
+COPY --link testfile $empty
+RUN [ "$(cat testfile)" == "contents0" ]
+`)
+
+	dir := integration.Tmpdir(
+		t,
+		fstest.CreateFile("Dockerfile", dockerfile, 0600),
+		fstest.CreateFile("testfile", []byte("contents0"), 0600),
+	)
+
+	c, err := client.New(sb.Context(), sb.Address())
+	require.NoError(t, err)
+	defer c.Close()
+
+	_, err = f.Solve(sb.Context(), c, client.SolveOpt{
+		LocalMounts: map[string]fsutil.FS{
+			dockerui.DefaultLocalNameDockerfile: dir,
+			dockerui.DefaultLocalNameContext:    dir,
+		},
+	}, nil)
+	require.NoError(t, err)
+}
+
 func testExportCacheLoop(t *testing.T, sb integration.Sandbox) {
 	integration.SkipOnPlatform(t, "windows")
 	workers.CheckFeatureCompat(t, sb, workers.FeatureCacheExport, workers.FeatureCacheImport, workers.FeatureCacheBackendLocal)