same default passwd in .env.example and docker-entrypoint

.env.example

@@ -1,7 +1,7 @@
 BEACON_URL=http://your-BN-ip:5052
 VALIDATOR_URL=http://your-VC-ip:5062
 API_TOKEN="get-it-from-'.lighthouse/validators/api-token.txt'"
-SESSION_PASSWORD="your-password"
+SESSION_PASSWORD="default-siren-password"
 SSL_ENABLED=true
 # don't change these when building the docker image
 PORT=3000