Use explicit UID/GID and show them in error message

docker/server/Dockerfile

@@ -10,7 +10,7 @@ VOLUME /data
 WORKDIR /data
 
 # running as a named unprivileged user
-RUN addgroup -S user && adduser -S user -G user
+RUN addgroup -S -g 1001 user && adduser -S -u 1001 user -G user
 RUN chown user:user /data
 USER user
 

docker/server/start.sh

@@ -6,7 +6,7 @@ then
     # Check if /data is writable
     if [ ! -w /data ]
     then
-        echo "Data directory (mounted) at '/data' is not writable for container user 'user:user', ZIM file can not be written."
+        echo "'/data' directory is not writable by '$(id -n -u):$(id -n -g)' ($(id -u):$(id -g)). ZIM file(s) can not be written."
         exit 1
     fi