CI Dockerfile: don't have a default value for ARG (#70)

Author: DilumAluthge

ci/Dockerfile

@@ -1,6 +1,7 @@
 FROM ghcr.io/kleinhenz/docker-slurm-cluster@sha256:c62b169970eaab879898a7df5950f888b0417147e06f8c0e32fd15087b22d9f2
 
-ARG JULIA_VERSION=1.11.2
+# We intentionally have no default value:
+ARG JULIA_VERSION
 
 RUN mkdir -p /home/docker/.local/opt/julia \
     && cd /home/docker/.local/opt/julia \