Add OSEM_RUBY_VERSION as build argument on Docker setup

Dockerfile

@@ -1,5 +1,6 @@
 FROM osem/base
 ARG CONTAINER_USERID
+ARG OSEM_RUBY_VERSION
 
 # Configure our user
 RUN usermod -u $CONTAINER_USERID osem

docker-compose.override.yml.example

@@ -4,3 +4,4 @@ services:
     build:
       args:
         CONTAINER_USERID: 13042
+        OSEM_RUBY_VERSION: 2.5.8

docker-compose.yml

@@ -10,6 +10,7 @@ services:
       context: .
       args:
         CONTAINER_USERID: 1000
+        OSEM_RUBY_VERSION: 2.5.0
     command: /osem/bin/osem-init.sh
     depends_on:
       - database