ci: build minimal and standard images (#135)

Build images without barman-cloud, to be used with backup plugins.

Other changes:

- Implement timestamp-based versioning for images
- Simplify build workflows for enhanced local testing and contribution
- Adopt OCI annotations and generate SBOMs for improved transparency

Closes #132

Signed-off-by: Francesco Canovai <francesco.canovai@enterprisedb.com>
Signed-off-by: Gabriele Bartolini <gabriele.bartolini@enterprisedb.com>
Signed-off-by: Marco Nenciarini <marco.nenciarini@enterprisedb.com>
Signed-off-by: Niccolò Fei <niccolo.fei@enterprisedb.com>
Co-authored-by: Gabriele Bartolini <gabriele.bartolini@enterprisedb.com>
Co-authored-by: Marco Nenciarini <marco.nenciarini@enterprisedb.com>
Co-authored-by: Niccolò Fei <niccolo.fei@enterprisedb.com>

BUILD.md

@@ -0,0 +1,129 @@
+# Building PostgreSQL Container Images for CloudNativePG
+
+This guide outlines the process for building PostgreSQL operand images for
+CloudNativePG using [Docker Bake](https://docs.docker.com/build/bake/) and a
+[GitHub workflow](.github/workflows/bake.yaml).
+
+The central component of this framework is the
+[Bake file (`docker-bake.hcl`)](docker-bake.hcl).
+
+## Prerequisites
+
+Ensure the following tools and components are available before proceeding:
+
+1. [Docker Buildx](https://github.com/docker/buildx): A CLI plugin for advanced
+image building.
+2. Build Driver for Multi-Architecture Images: For example, `docker-container`
+(see [Build Drivers](https://docs.docker.com/build/builders/drivers/) and
+["Install QEMU Manually"](https://docs.docker.com/build/building/multi-platform/#install-qemu-manually)).
+3. [Distribution Registry](https://distribution.github.io/distribution/):
+Formerly known as Docker Registry, to host and manage the built images.
+
+### Verifying Requirements
+
+To confirm your environment is properly set up, run:
+
+```bash
+docker buildx bake --check
+```
+
+If errors appear, you may need to switch to a different build driver. For
+example, use the following commands to configure a `docker-container` build
+driver:
+
+```bash
+docker buildx create \
+  --name docker-container \
+  --driver docker-container \
+  --use \
+  --driver-opt network=host \
+  --bootstrap
+```
+
+> *Note:* The `--driver-opt network=host` setting is required only for testing
+> when you push to a distribution registry listening on `localhost`.
+
+> *Note:* This page is not intended to serve as a comprehensive guide for
+> building multi-architecture images with Docker and Bake. If you encounter any
+> issues, please refer to the resources listed above for detailed instructions
+> and troubleshooting.
+
+## Default Target
+
+The `default` target in Bake represents a Cartesian product of the following
+dimensions:
+
+- **Base Image**
+- **Type** (e.g. `minimal` or `standard`)
+- **Platforms**
+- **PostgreSQL Versions**
+
+## Building Images
+
+To build PostgreSQL images using the `default` target — that is, for all the
+combinations of base image, format, platforms, and PostgreSQL versions — run:
+
+```bash
+docker buildx bake --push
+```
+
+> *Note:* The `--push` flag is required to upload the images to the registry.
+> Without it, the images will remain cached within the builder container,
+> making testing impossible.
+
+If you want to limit the build to a specific combination, you can specify the
+target in the `VERSION-TYPE-BASE` format. For example, to build an image for
+PostgreSQL 17 with the `minimal` format on the `bookworm` base image:
+
+```bash
+docker buildx bake --push postgresql-17-minimal-bookworm
+```
+
+You can also limit the build to a single platform, for example AMD64, with:
+
+```bash
+docker buildx bake --push --set "*.platform=linux/amd64"
+```
+
+The two can be mixed as well:
+
+```bash
+docker buildx bake --push \
+  --set "*.platform=linux/amd64" \
+  postgresql-17-minimal-bookworm
+```
+
+## The Distribution Registry
+
+The images must be pushed to any registry server that complies with the **OCI
+Distribution Specification**.
+
+By default, the build process assumes a registry server running locally at
+`localhost:5000`. To use a different registry, set the `registry` environment
+variable when executing the `docker` command, as shown:
+
+```bash
+registry=<REGISTRY_URL> docker buildx ...
+```
+
+## Local Testing
+
+You can test the image-building process locally if you meet the necessary
+[prerequisites](prerequisites).
+
+To do this, you'll need a local registry server. If you don't already have one,
+you can deploy a temporary, disposable [distribution registry](https://distribution.github.io/distribution/about/deploying/)
+with the following command:
+
+```bash
+docker run -d --rm -p 5000:5000 --name registry registry:2
+```
+
+This command runs a lightweight, temporary instance of the `registry:2`
+container on port `5000`.
+
+## Trademarks
+
+*[Postgres, PostgreSQL and the Slonik Logo](https://www.postgresql.org/about/policies/trademarks/)
+are trademarks or registered trademarks of the PostgreSQL Community Association
+of Canada, and used with their permission.*