README.md: Add description about postgresql-client switching behavior

Co-authored by: Steven Achilles <[email protected]>

Author: kkimurak

README.md

@@ -240,6 +240,29 @@ GitLab uses a database backend to store its data. You can configure this image t
 
 #### PostgreSQL
 
+**Important note:** This image is shipped with different versions of the `postgresql-client`.
+
+During the startup of the container, the major version of the database system is checked based on the specified connection destination. Only the version of the `postgresql-client`, that matches the major version of the Postgres database is used. All other versions of the `postgresql-client` are deleted at runtime.
+
+This behavior can be checked using the command `docker logs` and an output like the following should be available:
+
+````sh
+…
+Configuring gitlab::database
+- Installing postgresql client to avoid version mismatch on dumping
+-- Detected server version: 140007
+- Generating /home/git/.postgresqlrc
+14 postgresql:5432 gitlabhq_production
+- Uninstalling unused client(s): postgresql-client-12 postgresql-client-13 postgresql-client-15
+…
+````
+
+Please note furthermore, that only combatible versions of the `postgresql-client` to GitLab are shipped with this image. Currently these belogn to
+
+- `postgresql-client-12`,
+- `postgresql-client-13`,
+- `postgresql-client-14`,
+- and `postgresql-client-15`.
 *NOTE:* version 13.7.0 and later requires PostgreSQL version 12.x
 
 ##### External PostgreSQL Server