Fix Complement `TestCanRegisterAdmin` with workers, by adding Complement's shared registration secret. (#12819)

changelog.d/12819.misc

@@ -0,0 +1 @@
+Add Complement's shared registration secret to the Complement worker image. This fixes tests that depend on it.

docker/complement/conf-workers/workers-shared.yaml

@@ -5,6 +5,12 @@ enable_registration: true
 enable_registration_without_verification: true
 bcrypt_rounds: 4
 
+## Registration ##
+
+# Needed by Complement to register admin users
+# DO NOT USE in a production configuration! This should be a random secret.
+registration_shared_secret: complement
+
 ## Federation ##
 
 # trust certs signed by Complement's CA