refactor(docker): convert CMD instruction to exec form

Change CMD from multi-line shell form to JSON array exec form for better
signal handling and to follow Docker best practices. Consolidate the command
chain into a single line while maintaining the same functionality (migrate,
collectstatic, runserver).

dockerfile

@@ -17,8 +17,4 @@ COPY . .
 
 EXPOSE 8000
 
-CMD bash -c "
-  python manage.py migrate &&
-  python manage.py collectstatic --noinput &&
-  python manage.py runserver 0.0.0.0:8000
-"
+CMD ["bash", "-c", "python manage.py migrate && python manage.py collectstatic --noinput && python manage.py runserver 0.0.0.0:8000"]