dockerfile: avoid ignoring errors global arg expansion

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

frontend/dockerfile/dockerfile2llb/convert.go

@@ -226,7 +226,10 @@ func toDispatchState(ctx context.Context, dt []byte, opt ConvertOpt) (*dispatchS
 			info := argInfo{definition: metaArg, location: cmd.Location()}
 			if v, ok := opt.BuildArgs[metaArg.Key]; !ok {
 				if metaArg.Value != nil {
-					*metaArg.Value, info.deps, _ = shlex.ProcessWordWithMatches(*metaArg.Value, metaArgsToMap(optMetaArgs))
+					*metaArg.Value, info.deps, err = shlex.ProcessWordWithMatches(*metaArg.Value, metaArgsToMap(optMetaArgs))
+					if err != nil {
+						return nil, parser.WithLocation(err, cmd.Location())
+					}
 				}
 			} else {
 				metaArg.Value = &v

frontend/dockerfile/dockerfile_test.go

@@ -101,6 +101,7 @@ var allTests = integration.TestFuncs(
 	testPlatformArgsExplicit,
 	testExportMultiPlatform,
 	testQuotedMetaArgs,
+	testGlobalArgErrors,
 	testIgnoreEntrypoint,
 	testSymlinkedDockerfile,
 	testEmptyWildcard,
@@ -1291,6 +1292,46 @@ COPY --from=build /out .
 	require.Equal(t, "bar-box-foo", string(dt))
 }
 
+func testGlobalArgErrors(t *testing.T, sb integration.Sandbox) {
+	integration.SkipOnPlatform(t, "windows")
+	f := getFrontend(t, sb)
+
+	dockerfile := []byte(`
+ARG FOO=${FOO:?"custom error"}
+FROM busybox
+`)
+
+	dir := integration.Tmpdir(
+		t,
+		fstest.CreateFile("Dockerfile", dockerfile, 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.Error(t, err)
+
+	require.Contains(t, err.Error(), "FOO: custom error")
+
+	_, err = f.Solve(sb.Context(), c, client.SolveOpt{
+		FrontendAttrs: map[string]string{
+			"build-arg:FOO": "bar",
+		},
+		LocalMounts: map[string]fsutil.FS{
+			dockerui.DefaultLocalNameDockerfile: dir,
+			dockerui.DefaultLocalNameContext:    dir,
+		},
+	}, nil)
+	require.NoError(t, err)
+}
+
 func testMultiArgs(t *testing.T, sb integration.Sandbox) {
 	integration.SkipOnPlatform(t, "windows")
 	f := getFrontend(t, sb)