add guide for migrating database user to a non-superuser

docs/creation-of-nonsuperuser.md

@@ -0,0 +1,61 @@
+## Migrating existing superuser to a less privileged user
+
+Mattermost-docker used to use the initially created user while database initialization. This is being accomplished by using the
+`POSTGRES_USER` environment variable of the PostgreSQL Docker image. While this is convinient because it requires less setup steps
+it's best practice and desirable to us a less privileged user to connect to the database. The following steps should be safe and
+executable while Mattermost is running.
+
+**NOTE:** Commands with a **$** prefix denote those are executed as user, **#** as root and commands without a prefix are database commands.
+We assume the database name is *mattermost* and the database user *mmuser*.
+
+1. Find out the name or id of the PostgreSQL container
+To get either the name or the id of the running PostgeSQL container we can use `$ sudo docker ps`.
+
+2. Attaching to the database container
+`$ sudo docker exec -it POSTGRES_CONTAINER_NAME/ID /bin/sh`
+
+3. Connecting to the database
+```
+# psql DATABASE_NAME USERNAME
+e.g.
+# psql mattermost mmuser
+```
+
+4. Checking if the Mattermost user is a superuser
+The following PostgreSQL command will print a list of the present users and its attributes.
+```
+\du
+```
+A possible output can look like the following:
+
+```
+                                   List of roles
+ Role name |                         Attributes                         | Member of
+-----------+------------------------------------------------------------+-----------
+ mmuser    | Superuser, Create role, Create DB, Replication, Bypass RLS | {}
+```
+
+5. Creating a new `superuser` and changing existing role attributes
+**ATTENTION:** It's strongly recommended to create a database prior alteration. This can be done by stopping the database
+and backup the PostgreSQL data path at filesystem level and/or to use `pg_dumpall`. For this attach to the running PostgreSQL
+container described in step 2 and execute:
+```
+pg_dump -U mmuser -d mattermost > /var/lib/postgresql/data/BACKUP_MATTERMOST.sql
+```
+This dumps your *mattermost* database to the mounted directory, specified in the docker-compose.yml file.
+
+After your backup is done you can connect to the database (see step 3) and execute the following SQL queries:
+```
+CREATE ROLE superuser WITH BYPASSRLS REPLICATION CREATEDB CREATEROLE SUPERUSER LOGIN PASSWORD 'superuser_passwd';
+
+ALTER DATABASE mattermost OWNER TO superuser;
+ALTER DATABASE postgres OWNER TO superuser;
+ALTER DATABASE template0 OWNER TO superuser;
+ALTER DATABASE template1 OWNER TO superuser;
+
+GRANT ALL PRIVILEGES ON DATABASE mattermost to mmuser;
+
+ALTER ROLE mmuser NOBYPASSRLS NOREPLICATION NOCREATEDB NOCREATEROLE NOSUPERUSER;
+```
+
+Even though you can apply the changes in a non-downtime it's required to restart the containers.