diff --git a/frontend/dockerfile/builder/build.go b/frontend/dockerfile/builder/build.go
index c27590b7..76777eea 100644
--- a/frontend/dockerfile/builder/build.go
+++ b/frontend/dockerfile/builder/build.go
@@ -62,8 +62,10 @@ func Build(ctx context.Context, c client.Client) (*client.Result, error) {
 		localNameContext = v
 	}
 
+	forceLocalDockerfile := false
 	localNameDockerfile := DefaultLocalNameDockerfile
 	if v, ok := opts[keyNameDockerfile]; ok {
+		forceLocalDockerfile = true
 		localNameDockerfile = v
 	}
 
@@ -119,11 +121,14 @@ func Build(ctx context.Context, c client.Client) (*client.Result, error) {
 		llb.SharedKeyHint(localNameDockerfile),
 		dockerfile2llb.WithInternalName(name),
 	)
+
 	var buildContext *llb.State
 	isScratchContext := false
 	if st, ok := detectGitContext(opts[localNameContext]); ok {
-		src = *st
-		buildContext = &src
+		if !forceLocalDockerfile {
+			src = *st
+		}
+		buildContext = st
 	} else if httpPrefix.MatchString(opts[localNameContext]) {
 		httpContext := llb.HTTP(opts[localNameContext], llb.Filename("context"), dockerfile2llb.WithInternalName("load remote build context"))
 		def, err := httpContext.Marshal(marshalOpts...)
@@ -154,10 +159,13 @@ func Build(ctx context.Context, c client.Client) (*client.Result, error) {
 		if isArchive(dt) {
 			fileop := useFileOp(opts, &caps)
 			if fileop {
-				src = llb.Scratch().File(llb.Copy(httpContext, "/context", "/", &llb.CopyInfo{
+				bc := llb.Scratch().File(llb.Copy(httpContext, "/context", "/", &llb.CopyInfo{
 					AttemptUnpack: true,
 				}))
-				buildContext = &src
+				if !forceLocalDockerfile {
+					src = bc
+				}
+				buildContext = &bc
 			} else {
 				copyImage := opts[keyOverrideCopyImage]
 				if copyImage == "" {
@@ -166,13 +174,18 @@ func Build(ctx context.Context, c client.Client) (*client.Result, error) {
 				unpack := llb.Image(copyImage, dockerfile2llb.WithInternalName("helper image for file operations")).
 					Run(llb.Shlex("copy --unpack /src/context /out/"), llb.ReadonlyRootFS(), dockerfile2llb.WithInternalName("extracting build context"))
 				unpack.AddMount("/src", httpContext, llb.Readonly)
-				src = unpack.AddMount("/out", llb.Scratch())
-				buildContext = &src
+				bc := unpack.AddMount("/out", llb.Scratch())
+				if !forceLocalDockerfile {
+					src = bc
+				}
+				buildContext = &bc
 			}
 		} else {
 			filename = "context"
-			src = httpContext
-			buildContext = &src
+			if !forceLocalDockerfile {
+				src = httpContext
+			}
+			buildContext = &httpContext
 			isScratchContext = true
 		}
 	}
diff --git a/frontend/dockerfile/dockerfile_test.go b/frontend/dockerfile/dockerfile_test.go
index 337b5f46..cf261a9e 100644
--- a/frontend/dockerfile/dockerfile_test.go
+++ b/frontend/dockerfile/dockerfile_test.go
@@ -105,6 +105,7 @@ var fileOpTests = []integration.Test{
 	testCopyWildcards,
 	testCopyRelative,
 	testTarContext,
+	testTarContextExternalDockerfile,
 }
 
 var opts []integration.TestOpt
@@ -3834,6 +3835,72 @@ COPY foo /
 	require.NoError(t, err)
 }
 
+func testTarContextExternalDockerfile(t *testing.T, sb integration.Sandbox) {
+	f := getFrontend(t, sb)
+	isFileOp := getFileOp(t, sb)
+
+	foo := []byte("contents")
+
+	buf := bytes.NewBuffer(nil)
+	tw := tar.NewWriter(buf)
+	err := tw.WriteHeader(&tar.Header{
+		Name:     "foo",
+		Typeflag: tar.TypeReg,
+		Size:     int64(len(foo)),
+		Mode:     0644,
+	})
+	require.NoError(t, err)
+	_, err = tw.Write(foo)
+	require.NoError(t, err)
+	err = tw.Close()
+	require.NoError(t, err)
+
+	dockerfile := []byte(`
+FROM scratch
+COPY foo bar
+`)
+	dir, err := tmpdir(
+		fstest.CreateFile("Dockerfile", dockerfile, 0600),
+	)
+	require.NoError(t, err)
+	defer os.RemoveAll(dir)
+
+	c, err := client.New(context.TODO(), sb.Address())
+	require.NoError(t, err)
+	defer c.Close()
+
+	up := uploadprovider.New()
+	url := up.Add(buf)
+
+	// repeat with changed default args should match the old cache
+	destDir, err := ioutil.TempDir("", "buildkit")
+	require.NoError(t, err)
+	defer os.RemoveAll(destDir)
+
+	_, err = f.Solve(context.TODO(), c, client.SolveOpt{
+		FrontendAttrs: map[string]string{
+			"build-arg:BUILDKIT_DISABLE_FILEOP": strconv.FormatBool(!isFileOp),
+			"context":                           url,
+			"dockerfilekey":                     builder.DefaultLocalNameDockerfile,
+		},
+		Session: []session.Attachable{up},
+		LocalDirs: map[string]string{
+			builder.DefaultLocalNameDockerfile: dir,
+		},
+		Exports: []client.ExportEntry{
+			{
+				Type:      client.ExporterLocal,
+				OutputDir: destDir,
+			},
+		},
+	}, nil)
+	require.NoError(t, err)
+
+	dt, err := ioutil.ReadFile(filepath.Join(destDir, "bar"))
+	require.NoError(t, err)
+	require.Equal(t, string(dt), "contents")
+}
+
 func tmpdir(appliers ...fstest.Applier) (string, error) {
 	tmpdir, err := ioutil.TempDir("", "buildkit-dockerfile")
 	if err != nil {