fix!: reverse proxy to sub-directory (#3483)

from this commit, if you want reverse proxy to sub-directory like `alist` with `nginx`, you need config:

```nginx
location /alist/ {
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header X-Forwarded-Proto $scheme;
    proxy_set_header Host $http_host;
    proxy_set_header X-Real-IP $remote_addr;
    proxy_set_header Range $http_range;
    proxy_set_header If-Range $http_if_range;
    proxy_redirect off;
    proxy_pass http://127.0.0.1:5244/alist/;
    # the max size of file to upload
    client_max_body_size 20000m;
}
```

internal/bootstrap/config.go

@@ -2,8 +2,10 @@ package bootstrap
 
 import (
 	"crypto/tls"
+	"net/url"
 	"os"
 	"path/filepath"
+	"strings"
 
 	"github.com/alist-org/alist/v3/cmd/flags"
 	"github.com/alist-org/alist/v3/drivers/base"
@@ -79,6 +81,7 @@ func InitConfig() {
 	if conf.Conf.TlsInsecureSkipVerify {
 		base.RestyClient = base.RestyClient.SetTLSClientConfig(&tls.Config{InsecureSkipVerify: true})
 	}
+	initURL()
 }
 
 func confFromEnv() {
@@ -93,3 +96,14 @@ func confFromEnv() {
 		log.Fatalf("load config from env error: %+v", err)
 	}
 }
+
+func initURL() {
+	if !strings.Contains(conf.Conf.SiteURL, "://") {
+		conf.Conf.SiteURL = utils.FixAndCleanPath(conf.Conf.SiteURL)
+	}
+	u, err := url.Parse(conf.Conf.SiteURL)
+	if err != nil {
+		utils.Log.Fatalf("can't parse site_url: %+v", err)
+	}
+	conf.URL = u
+}