feat: add PostgreSQL 17beta1 images (#107)

Adding documentation on the project's structure and on how to add a new Postgres Beta version.

Signed-off-by: Gabriele Bartolini <gabriele.bartolini@enterprisedb.com>
Signed-off-by: Niccolò Fei <niccolo.fei@enterprisedb.com>
Co-authored-by: Niccolò Fei <niccolo.fei@enterprisedb.com>

Debian/README.md

@@ -0,0 +1,56 @@
+# Debian-based Container Images
+
+This folder contains the `Dockerfile` manifests needed to build Debian-based
+PostgreSQL operand container images for use with CloudNativePG.
+
+The images are based on the official [PostgreSQL images on
+DockerHub](https://hub.docker.com/_/postgres).
+
+The folder includes:
+
+- Directories for each supported PostgreSQL version.
+- [Image catalog
+  files](https://cloudnative-pg.io/documentation/current/image_catalog/) for
+each Debian version.
+- Templates for the main `Dockerfile` and the beta version `Dockerfile`
+  (typically identical, except for some extensions that are not ready for the
+  new major release of PostgreSQL).
+- The `requirements.in` file required to build Barman Cloud images. (Note: This
+  file will be removed once a Barman Cloud plugin supporting CNPG-I is
+  distributed.)
+- The main update script.
+
+## Adding a New Beta Version
+
+To add a new beta version, follow these steps:
+
+1. Create a new issue in the ["postgres-containers" project](https://github.com/cloudnative-pg/postgres-containers)
+   with the title "Add PostgreSQL XX beta1 images".
+2. Clone the `postgres-containers` repository.
+3. Create a new branch named after the issue ID (e.g., `dev/YYY`).
+4. Create the `Debian/XX/` directory.
+5. Identify the latest Debian version name (e.g., `bookworm`).
+6. Run `Debian/update.sh XX -d bookworm` which will create a
+   `.versions.json` file into the `Debian/XX/bookworm/` directory.
+7. Add the new directory to your commit and push the changes.
+8. Run the `Automatic updates` action on the branch and wait for it to complete
+   (this will add a commit to the branch).
+9. Submit a pull request.
+
+
+## Troubleshooting
+
+### Common issues
+
+* Error while running `update.sh`
+
+```
+sed: can't read requirements.txt: No such file or directory
+cat: requirements.txt: No such file or directory
+rm: cannot remove 'requirements.txt': No such file or directory
+```
+
+If a similar error appears, the reason is that you are missing the
+`pip-compile` utility that's needed to build the python requirements
+for Barman Cloud. (Note: This requirement will be removed once a
+Barman Cloud plugin supporting CNPG-I is distributed.)