Fix and explain user buildargs (#49)

* expose GID AND UID as optional build args

* Give examples on how to use the build-time args in docker-compose

* Prevent overwriting of build args

* Warn about using regular docker run

docker-compose.yml

@@ -15,7 +15,6 @@ services:
       - ./postgres:/var/lib/postgresql/data
 
   web:
-    build: .
     image: pleroma
     container_name: pleroma_web
     healthcheck:
@@ -27,6 +26,14 @@ services:
     restart: always
     ports:
       - '4000:4000'
+    build:
+      context: .
+      # Feel free to remove or override this section
+      # See 'Build-time variables' in README.md
+      args:
+        - "UID=1000"
+        - "GID=1000"
+        - "PLEROMA_VER=v2.4.2"
     volumes:
       - ./uploads:/var/lib/pleroma/uploads
       - ./static:/var/lib/pleroma/static