fix(docker): handle existing GID/UID 1000 in Ubuntu 24.04 base image

Resolve Docker build failure when creating appuser with GID/UID 1000

The Ubuntu 24.04 CUDA base image already contains a user/group with GID 1000, 
causing the Docker build to fail with "groupadd: GID '1000' already exists".

Changes made:
- Add graceful handling for existing GID 1000 using `|| true` pattern
- Add graceful handling for existing UID 1000 to prevent user creation conflicts  
- Ensure /home/appuser directory creation with explicit mkdir -p
- Add explicit ownership assignment (chown 1000:1000) regardless of user creation outcome
- Suppress stderr output from groupadd/useradd commands to reduce build noise

This fix ensures the Docker build succeeds across different CUDA base image versions 
while maintaining the intended UID/GID mapping (1000:1000) required by the entrypoint 
script's permission management system.

The container will now build successfully and the entrypoint script will still be 
able to perform proper user/group remapping at runtime via PUID/PGID environment 
variables as designed.

Fixes build error:

Dockerfile

@@ -33,9 +33,11 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
  && ln -sf /usr/bin/python3.12 /usr/bin/python3 \
  && rm -rf /var/lib/apt/lists/*
 
-# Create runtime user/group
-RUN groupadd --gid 1000 appuser \
- && useradd --uid 1000 --gid 1000 --create-home --shell /bin/bash appuser
+# Create runtime user/group (handle existing GID/UID gracefully)
+RUN (groupadd --gid 1000 appuser 2>/dev/null || true) \
+ && (useradd --uid 1000 --gid 1000 --create-home --shell /bin/bash appuser 2>/dev/null || true) \
+ && mkdir -p /home/appuser \
+ && chown -R 1000:1000 /home/appuser
 
 # Workdir
 WORKDIR /app/ComfyUI