Allow scaling joex with docker-compose up --scale

Container name can't be hard coded and each joex instance needs a unique
name. Since Docker always sets the `HOSTNAME` variable and these are
unique, we can just interpolate the hostname into the joex app
identifier, to avoid creating multiple config files.

docker/docspell.conf

@@ -27,7 +27,8 @@ docspell.server {
 }
 
 docspell.joex {
-  base-url = "http://joex:7878"
+  app-id = "joex-"${HOSTNAME}
+  base-url = "http://"${HOSTNAME}":7878"
   bind {
     address = "0.0.0.0"
   }