Docker build machinery: a little bit of documentation

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+# Docker files for our test machinery
+
+**These files are not distribution files for running BIRD. We currently
+do not supply official BIRD images and do not prepare any Docker files ourselves.**
+
+We build for the major Linux distributions, FreeBSD and OpenBSD. If you feel
+that your favourite Linux distribution is missing, feel free to send a patch.
+
+Where to add your favourite Linux distribution:
+
+- add an appropriate dockerfile here
+- add `docker-*` job in `.gitlab-ci.yml`
+- add `build-*` job in `.gitlab-ci.yml`
+- add `pkg-*` job in `.gitlab-ci.yml`
+
+Our build machinery needs at least Python 3.6 because of `beautifulsoup4`.
+There is a hack for older distributions, installing an older version of
+that dependency which works also with an older Python.
+
+## Debian-based distributions
+
+We support Debian between oldoldstable and testing. If not, poke us.
+
+We support Ubuntu LTS at least 5 years old and non-LTS before EOL. After EOL,
+the non-LTS package repositories tend to disappear quite quickly so we don't
+have resources to build against.
+
+## RedHat-based distributions
+
+We support OpenSUSE, Fedora and CentOS. If you are missing your favourite new
+release, poke us. We are discontinuing the old releases as they stop working.
+
+The current support for CentOS 7 and 8 has been paid for and we may drop it without
+further notice at the exact moment the customer stops using it.
+
+## Any other based distributions
+
+We currently don't package for Arch or Gentoo. Contributions are open, please
+refer to `CONTRIBUTING.md` for further information.
+
+You may also need to send a patch to [APKG](https://gitlab.nic.cz/packaging/apkg)
+to facilitate the package building. Yet, if you wish to just test and check
+builds without packaging, feel free to send the patch anyway.
+
+## FreeBSD and OpenBSD
+
+These are not built in Docker but in proper virtuals in QEMU, refer to
+<https://gitlab.nic.cz/labs/bird-tools/-/tree/master/birdlab-tmp>