update PR template for customer replica repo (#899)

Author: coury-clark

.github/PULL_REQUEST_TEMPLATE.md

@@ -8,6 +8,7 @@
   this repository. If uneeded, add link or explanation of why it is not needed here.
 -->
 * [ ] Sister [deploy-sourcegraph](https://github.com/sourcegraph/deploy-sourcegraph) change:
+* [ ] Sister [customer-replica](https://github.com/sourcegraph/deploy-sourcegraph-docker-customer-replica-1) change (if necessary, for any changes affecting pure-docker or configuration):
 * [ ] All images have a valid tag and SHA256 sum
 ### Test plan