[configurator] pin ubuntu image to fix build on mac

Author: Anviking

compose/configurator/Dockerfile

@@ -1,7 +1,7 @@
 # Blink Labs images are built from source on Debian Bookworm
 FROM ghcr.io/blinklabs-io/cardano-node:latest AS cardano-node
 
-FROM ubuntu
+FROM ubuntu@sha256:7c06e91f61fa88c08cc74f7e1b7c69ae24910d745357e0dfe1d2c0322aaf20f9
 
 USER root
 # Install packages required in build stage