[seafile-server] Document how official releases were built

[zhanghai]

Hi, I'm trying to build seafile-server from source and I came across seafile-rpi's build3.sh which invokes seafile-server's build-server.py, and it seems to generate something similar to the offcial download https://download.seadrive.org/seafile-server_8.0.1_x86-64.tar.gz.

I'm building it with GitHub actions at zhanghai/seafile-server-github-actions-build. However, one strange thing is that although I explicitly set python to 3.6 with GitHub's official action setup-python, the result artifact at https://github.com/zhanghai/seafile-server-github-actions-build/releases/tag/v8.0.1 still contains python2.7 under seafile/lib, unlike the official build which contains python3.6 under seafile/lib64.

Meanwhile, it was announced that ccnet-server was removed in seafile-server 8.0.1, and libccnet.so indeed disappeared from seafile/lib in the official build, but my GitHub Actions build still contains ccnet in a number of places. So it seems the build-server.py in seafile-server is outdated.

Could you provide more information on how exactly was the official 8.0.1 release built (e.g. the latest build script & dependency versions)?