🐛(docker) switch CMD form from Shell to Exec

`backend-development` and `backend-production` CMD syntaxes were
using a Shell Form. Shell form prevented Unix signals from reaching
our container correctly, such as SIGTERM. Also, the shell process
ends up being the PID 1, instead of our Python scripts.

Docker recommends to use the exec form whenever possible.

Dockerfile

@@ -136,4 +136,4 @@ COPY --from=link-collector ${IMPRESS_STATIC_ROOT} ${IMPRESS_STATIC_ROOT}
 COPY --from=mail-builder /mail/backend/core/templates/mail /app/core/templates/mail
 
 # The default command runs gunicorn WSGI server in impress's main module
-CMD gunicorn -c /usr/local/etc/gunicorn/impress.py impress.wsgi:application
+CMD ["gunicorn", "-c", "/usr/local/etc/gunicorn/impress.py", "impress.wsgi:application"]