Add new "docker-ensure-initdb.sh" script

This mimics the behavior of `docker-entrypoint.sh` before it starts the PostgreSQL server.

It has three main goals/uses:

  1. (most importantly) as an example of how to use "docker-entrypoint.sh" to extend/reuse the initialization behavior

  2. ("docker-ensure-initdb.sh") as a Kubernetes "init container" to ensure the provided database directory is initialized; see also "startup probes" for an alternative solution
       (no-op if database is already initialized)

  3. ("docker-enforce-initdb.sh") as part of CI to ensure the database is fully initialized before use
       (error if database is already initialized)

Author: tianon

.gitattributes

@@ -1,3 +1,4 @@
-/*/**/Dockerfile            linguist-generated
-/*/**/docker-entrypoint.sh  linguist-generated
-/Dockerfile*.template       linguist-language=Dockerfile
+/*/**/Dockerfile               linguist-generated
+/*/**/docker-ensure-initdb.sh  linguist-generated
+/*/**/docker-entrypoint.sh     linguist-generated
+/Dockerfile*.template          linguist-language=Dockerfile