Fix: Docker file permissions for non-root user (#17)

Container tests were failing with 'Permission denied' errors when trying
to read configuration files (.env, models.json) and application files.

Root cause: Files were copied as root user before switching to 'node' user,
resulting in the node user lacking read permissions.

Solution: Added explicit ownership and permission setting before USER switch:
- chown -R node:node /app - Set node as owner of all files
- chmod -R u+rw,go+r /app - Grant read/write to user, read to others
- chmod +x /app/src/server.js - Make server.js executable

This ensures consistent behavior across different Docker versions and
build environments.

All 303 tests now passing.

Author: sveneisenschmidt

docker/Dockerfile.build

@@ -21,6 +21,11 @@ COPY src/ ./src/
 COPY docker/.env ./.env
 COPY docker/models.json ./models.json
 
+# Set correct ownership and permissions for node user
+RUN chown -R node:node /app && \
+    chmod -R u+rw,go+r /app && \
+    chmod +x /app/src/server.js
+
 # Switch to non-root user for security
 USER node