Allow packagers to set WebUI root to custom path (#5809)

Co-authored-by: qwerty287 <80460567+qwerty287@users.noreply.github.com>
Co-authored-by: Robert Kaussow <xoxys@rknet.org>

docs/docs/92-development/10-packaging.md

@@ -0,0 +1,18 @@
+# Packaging
+
+If you repackage it, we encourage to build from source, which requires internet connection.
+
+For offline builds, we also offer a tarball with all vendored dependencies and a pre-built web UI
+on the [release page](https://github.com/woodpecker-ci/woodpecker/releases).
+
+## Distribute web UI in own directory
+
+If you do not want to embed the web UI in the binary, you can compile a custom root path for the web UI into the binary.
+
+Add `external_web` to the tags and use the build flag `-X go.woodpecker-ci.org/woodpecker/v3/web.webUIRoot=/some/path` to set a custom path.
+
+Example: <!-- cspell:ignore webui -->
+
+```sh
+go build -tags 'external_web' -ldflags '-s -w -extldflags "-static" -X go.woodpecker-ci.org/woodpecker/v3/version.Version=3.12.0 -X go.woodpecker-ci.org/woodpecker/v3/web.webUIRoot=/nix/store/maaajlp8h5gy9zyjgfhaipzj07qnnmrl-woodpecker-WebUI-3.12.0' -o dist/woodpecker-server go.woodpecker-ci.org/woodpecker/v3/cmd/server
+```

web/web.go

@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//go:build !external_web
+
 package web
 
 import (

web/web_external.go

@@ -0,0 +1,54 @@
+// Copyright 2025 Woodpecker Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//go:build external_web
+
+package web
+
+import (
+	"fmt"
+	"io"
+	"net/http"
+	"os"
+)
+
+var webUIRoot string // do not forget to set it at build time
+
+func HTTPFS() (http.FileSystem, error) {
+	if stat, err := os.Stat(webUIRoot); err != nil {
+		return nil, fmt.Errorf("compiled in WebUI root path '%s' does not exist: %w", webUIRoot, err)
+	} else if !stat.IsDir() {
+		return nil, fmt.Errorf("compiled in WebUI root path '%s' exist but is no directory", webUIRoot)
+	}
+	return http.Dir(webUIRoot), nil
+}
+
+func Lookup(path string) (buf []byte, err error) {
+	httpFS, err := HTTPFS()
+	if err != nil {
+		return nil, err
+	}
+	file, err := httpFS.Open(path)
+	if err != nil {
+		return nil, err
+	}
+	defer file.Close()
+
+	buf, err = io.ReadAll(file)
+	if err != nil {
+		return nil, err
+	}
+
+	return buf, nil
+}