use sha for registration authority image

- this image is the only working version i can find and i dont want the tag to be updated

docker-compose/identity-authority.yml

@@ -44,7 +44,7 @@ services:
       - snet-asclepios
 
   registration-authority:
-    image: registry.gitlab.com/asclepios-project/registration-authority-cpabe:edit-psw-fixes
+    image: registry.gitlab.com/asclepios-project/registration-authority-cpabe@sha256:b9c291b0caa9ca800df0c92949f6d6a69faefe3e603697a6f590cdde40e39333
     container_name: registration-authority
     environment:
       MODE: ${REGISTRATION_AUTHORITY_MODE}