Update 'Hello Postgres cluster' example to replace deprecated `bitnamilegacy` image

Hi,

I was exploring your excellent collection of SQL scripts and Docker examples, and I noticed that the `running_pg_cluster_in_docker/1. Hello Postgres cluster/docker-compose.yml` file uses the `bitnamilegacy/postgresql-repmgr:17.6.0` Docker image.

**The Problem**
As the namespace implies, `bitnamilegacy` contains deprecated artifacts that are no longer actively maintained. Using this legacy repository in an educational/reference example is risky because these images do not receive necessary updates or CVE security patches. Users copying this `docker-compose.yml` for their own projects will unknowingly deploy an unmaintained database cluster.

**Proposed Solution**
I suggest switching to **`soldevelo/postgresql-repmgr`**.
This image is a free, actively maintained, and fully open-source drop-in replacement.

Crucially, it is perfectly compatible with your current example:
* It preserves the exact same environment variables, `repmgr` logic, and internal paths.
* Your Docker Compose setup will continue to work immediately without any structural changes, but it will be backed by a secure, actively supported image.

*(Note: SolDevelo maintains a growing library of open-source, Bitnami-compatible images to help the community move away from deprecated or paywalled artifacts. You can find the source code and other images here: [https://github.com/SolDevelo/containers](https://github.com/SolDevelo/containers))*

**Suggested Changes in `running_pg_cluster_in_docker/1. Hello Postgres cluster/docker-compose.yml`:**
```yaml
# Old
container_name: postgres_1
image: docker.io/bitnamilegacy/postgresql-repmgr:17.6.0
...
container_name: postgres_2
image: docker.io/bitnamilegacy/postgresql-repmgr:17.6.0

# New
container_name: postgres_1
image: docker.io/soldevelo/postgresql-repmgr:17.6.0
...
container_name: postgres_2
image: docker.io/soldevelo/postgresql-repmgr:17.6.0
```

I would be happy to submit a Pull Request to update this reference so your example remains robust and follows best security practices. Would you be open to that?

Open running_pg_cluster_in_docker/1. Hello Postgres cluster/docker-compose.yml and inspect the two PostgreSQL service image references. Update both references to soldevelo/postgresql-repmgr:17.6.0, then run the existing Docker Compose example to confirm the cluster still starts with its current configuration.

Written by the indexing model from the issue text.