Remove realms dump from source control

* Note: the docker compose command automatically creates the bind mounts if they don't exist. This however can lead to permission problems and thus we use a .gitkeep file to preserve the directory's ownership & permissions

.gitignore

 # the mapped volume for the migration file
 # current setup maps all the tmp files, so exclude them
 export/*
-
-# except the migration file(s)
-!export/*realms*.json
+!export/.gitkeep
 
 # the secrets file
 .env