doc: simplified how to use the base URL feature

doc/INSTALL.md

@@ -81,10 +81,10 @@ and Docker Compose (version 1.6.0 or greater) already installed.
     Some users may wish to access the service at a different base URI, such
     as `http://example.com/szuru/`, commonly when sharing multiple HTTP
     services on one domain using a reverse proxy. In this case, simply set
-    `BASE_URL="/szuru/"` in the frontend container (unless you are hosting your
-    data on a different domain).
+    `BASE_URL="/szuru/"` in your `.env` file.
 
-    You should set your reverse proxy to proxy `http(s)://example.com/szuru` to
+    Note that this will require a reverse proxy to function. You should set
+    your reverse proxy to proxy `http(s)://example.com/szuru` to
     `http://<internal IP or hostname of frontend container>/`. For an NGINX
     reverse proxy, that will appear as:
 

doc/example.env

@@ -10,6 +10,10 @@ BUILD_INFO=latest
 # otherwise the port specified here will be publicly accessible
 PORT=8080
 
+# URL base to run szurubooru under
+# See "Additional Features" section in INSTALL.md
+BASE_URL=/
+
 # Directory to store image data
 MOUNT_DATA=/var/local/szurubooru/data
 

docker-compose.yml

@@ -37,6 +37,7 @@ services:
       - api
     environment:
       BACKEND_HOST: api
+      BASE_URL:
     volumes:
       - "${MOUNT_DATA}:/data:ro"
     ports: