[Fix] Add ldconfig and library path configuration to Dockerfile

[Fix] Add ldconfig and library path configuration to Dockerfile

The rspamd shared libraries are installed in /usr/lib/rspamd/ which is not
in the default dynamic linker search path. This causes the error:
  librspamd-server.so: cannot open shared object file

Fixed by:
- Adding /usr/lib/rspamd to /etc/ld.so.conf.d/rspamd.conf
- Running ldconfig to update the dynamic linker cache

This ensures all rspamd shared libraries are found at runtime.