Update complement to use setuptools entrypoints (#328)

Complement side of the synapse change in #12118

Author: David Robertson

dockerfiles/Synapse.Dockerfile

@@ -24,7 +24,7 @@ COPY keys/* /ca/
 RUN openssl genrsa -out /conf/server.tls.key 2048
 
 # generate a signing key
-RUN generate_signing_key.py -o /conf/server.signing.key
+RUN generate_signing_key -o /conf/server.signing.key
 
 WORKDIR /data