dockerfile: apply dockerignore on loading local contexts

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

frontend/dockerfile/builder/build.go

@@ -863,7 +863,44 @@ func contextByName(ctx context.Context, c client.Client, name string, platform *
 		}
 		return st, nil, nil
 	case "local":
-		st := llb.Local(vv[1], llb.WithCustomName("[context "+name+"] load from client"), llb.SessionID(c.BuildOpts().SessionID), llb.SharedKeyHint("context:"+name))
+		st := llb.Local(vv[1],
+			llb.SessionID(c.BuildOpts().SessionID),
+			llb.FollowPaths([]string{dockerignoreFilename}),
+			llb.SharedKeyHint("context:"+name+"-"+dockerignoreFilename),
+			llb.WithCustomName("[context "+name+"] load "+dockerignoreFilename),
+			llb.Differ(llb.DiffNone, false),
+		)
+		def, err := st.Marshal(ctx)
+		if err != nil {
+			return nil, nil, err
+		}
+		res, err := c.Solve(ctx, client.SolveRequest{
+			Evaluate:   true,
+			Definition: def.ToPB(),
+		})
+		if err != nil {
+			return nil, nil, err
+		}
+		ref, err := res.SingleRef()
+		if err != nil {
+			return nil, nil, err
+		}
+		dt, _ := ref.ReadFile(ctx, client.ReadRequest{
+			Filename: dockerignoreFilename,
+		}) // error ignored
+		var excludes []string
+		if len(dt) != 0 {
+			excludes, err = dockerignore.ReadAll(bytes.NewBuffer(dt))
+			if err != nil {
+				return nil, nil, err
+			}
+		}
+		st = llb.Local(vv[1],
+			llb.WithCustomName("[context "+name+"] load from client"),
+			llb.SessionID(c.BuildOpts().SessionID),
+			llb.SharedKeyHint("context:"+name),
+			llb.ExcludePatterns(excludes),
+		)
 		return &st, nil, nil
 	case "input":
 		inputs, err := c.Inputs(ctx)

frontend/dockerfile/dockerfile_test.go

@@ -5471,7 +5471,7 @@ func testNamedLocalContext(t *testing.T, sb integration.Sandbox) {
 FROM busybox AS base
 RUN cat /etc/alpine-release > /out
 FROM scratch
-COPY --from=base /out /
+COPY --from=base /o* /
 `)
 
 	dir, err := tmpdir(
@@ -5484,6 +5484,8 @@ COPY --from=base /out /
 
 	dir2, err := tmpdir(
 		fstest.CreateFile("out", outf, 0600),
+		fstest.CreateFile("out2", outf, 0600),
+		fstest.CreateFile(".dockerignore", []byte("out2\n"), 0600),
 	)
 	require.NoError(t, err)
 	defer os.RemoveAll(dir2)
@@ -5515,6 +5517,10 @@ COPY --from=base /out /
 	dt, err := ioutil.ReadFile(filepath.Join(destDir, "out"))
 	require.NoError(t, err)
 	require.True(t, len(dt) > 0)
+
+	_, err = ioutil.ReadFile(filepath.Join(destDir, "out2"))
+	require.Error(t, err)
+	require.True(t, errors.Is(err, os.ErrNotExist))
 }
 
 func testNamedInputContext(t *testing.T, sb integration.Sandbox) {