Update compose-file-v2.md

Updated Documentation section ARGS added a note about the fact that ARG defined befor FROM is not there after FROM without explicit adding it again after FROM

compose/compose-file/compose-file-v2.md

@@ -196,6 +196,10 @@ or a list:
       args:
         - buildno=1
         - gitcommithash=cdc3b19
+        
+> **Note**: if you specify `ARG` in your Dockerfile befor the `FROM` instruction 
+> then the `ARG` will not be available in the buld instructions under `FROM`
+> if you need a argument in both places simply place it again under the `FROM` instruction
 
 You can omit the value when specifying a build argument, in which case its value
 at build time is the value in the environment where Compose is running.