fix: use groupadd/useradd instead of addgroup/adduser for Ubuntu 24.04

Ubuntu 24.04 doesn't have addgroup/adduser commands installed by default.
Switched to groupadd/useradd which are standard on all Ubuntu versions.

Author: mr-karan

Dockerfile

@@ -9,8 +9,8 @@ COPY config.sample.toml config.toml
 ARG GULLAK_GID="999"
 ARG GULLAK_UID="999"
 
-RUN addgroup --system --gid $GULLAK_GID gullak && \
-    adduser --uid $GULLAK_UID --system --ingroup gullak gullak && \
+RUN groupadd --system --gid $GULLAK_GID gullak && \
+    useradd --uid $GULLAK_UID --system --gid gullak --no-create-home gullak && \
     chown -R gullak:gullak /app
 
 USER gullak