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/docker-compose.yml

@@ -15,7 +15,6 @@ services:
 
   joex:
     image: eikek0/docspell:joex-LATEST
-    container_name: docspell-joex
     restart: unless-stopped
     env_file: ./.env
     ports: