introduce the INSTANCE_URL .env parameter

filed/.env.example

@@ -6,4 +6,6 @@ REDIS_HOST=redis
 REDIS_PORT=6379
 REDIS_PREFIX=bfile-
 
-USERCONTENT_DIR=/opt/user_uploads
+USERCONTENT_DIR=/opt/user_uploads
+
+INSTANCE_URL=http://localhost

filed/src/env.rs

@@ -18,7 +18,8 @@ pub struct Env {
     pub logging: bool,
     pub listen: SocketAddr,
     pub redis: Redis,
-    pub filedir: String
+    pub filedir: String,
+    pub instanceurl: String
 }
 
 fn get_var<T: Into<String>, O: From<String>>(name: T) -> Result<O, String> {
@@ -51,7 +52,8 @@ pub fn loadenv() -> Result<Env, Box<dyn std::error::Error>> {
                     return Err(format!("USERCONTENT_DIR is set to \"{}\", which exists but is not a directory!", &spath).into())
                 }
                 spath
-            }
+            },
+            instanceurl: get_var("INSTANCE_URL")?
         }
     )
 }