Document package building (gardenlinux#2185)

Co-authored-by: Vincent Riesop <v.riesop@sap.com>

docs/01_developers/build_packages.md

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+Garden Linux packages are built using [the `package-build` scripts](https://github.com/gardenlinux/package-build).
+
+Each package has it's own GitHub repository following the naming convention `package-{package_name}`.
+Those packages are picked up by [the workflows of the 'repo'](https://github.com/gardenlinux/repo).
+
+As a trivial example, have a look at [`gardenlinux/package-containerd`](https://github.com/gardenlinux/package-containerd).
+
+As a non-trivial example have a look at [`gardenlinux/package-openssh`](https://github.com/gardenlinux/package-openssh).
+
+If possible, we use the binary packages as provided by debian.
+Only in cases where it is needed to make a change, we rebuild the package and thus create a GitHub repository `package-{package_name}`.
+
+There are different types of package rebuilds:
+
+1. Source from debian:
+  - use case: apply Garden Linux-specific patches on top
+  - use case: rebuild with a specific build environment (e.g. golang, glibc, gcc, ...), for example when newer compiler/runtime is needed
+  - use case: a package we include was (temporarily) removed from debian testing
+    - this happens from time to time and is to be expected
+2. Source from upstream project:
+  - use case: debian does not maintain a package for the given software we need
+  - use case: debian maintains an old version only, we need to create a package for a newer version available in upstream
+3. Native source:
+  - use case: source code is developed/maintained by the Garden Linux developers, so debian does not package it