Add backup instructions for multi container and debian

docs/admin/backup.rst

+Backup your Funkwhale instance
+==============================
+
+.. note::
+
+    Before upgrading your instance, we strongly advise you to make at least a database backup. Ideally, you should make a full backup, including the database and the media files.
+
+
+Docker setup
+------------
+
+If you've followed the setup instructions in :doc:`../installation/docker`, here is the backup path:
+
+Multi-container installation
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Backup the db
+^^^^^^^^^^^^^
+
+On docker setups, you have to ``pg_dumpall`` in container ``funkwhale_postgres_1``:
+
+.. code-block:: shell
+
+   docker exec -t funkwhale_postgres_1 pg_dumpall -c -U postgres > dump_`date +%d-%m-%Y"_"%H_%M_%S`.sql
+
+Backup the media files
+^^^^^^^^^^^^^^^^^^^^^^
+
+To backup docker data volumes, as the volumes are bound mounted to the host, the ``rsync`` way would go like this:
+
+.. code-block:: shell
+
+    rsync -avzhP /srv/funkwhale/data/media /path/to/your/backup/media
+    rsync -avzhP /srv/funkwhale/data/music /path/to/your/backup/music
+
+
+Backup the configuration files
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+On docker setups, the configuration file is located at the root level:
+
+.. code-block:: shell
+
+    rsync -avzhP /srv/funkwhale/.env /path/to/your/backup/.env
+
+
+Non-docker setup
+----------------
+
+Backup the db
+^^^^^^^^^^^^^
+
+On non-docker setups, you have to ``pg_dump`` as user ``postgres``:
+
+.. code-block:: shell
+
+   sudo -u postgres -H pg_dump funkwhale > /path/to/your/backup/dump_`date +%d-%m-%Y"_"%H_%M_%S`.sql
+
+Backup the media files
+^^^^^^^^^^^^^^^^^^^^^^
+
+A simple way to backup your media files is to use ``rsync``:
+
+.. code-block:: shell
+
+    rsync -avzhP /srv/funkwhale/data/media /path/to/your/backup/media
+    rsync -avzhP /srv/funkwhale/data/music /path/to/your/backup/music
+
+Backup the configuration files
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+.. code-block:: shell
+
+    rsync -avzhP /srv/funkwhale/config/.env /path/to/your/backup/.env
+
+.. note::
+   You may also want to backup your proxy configuration file.
+
+   For frequent backups, you may want to use deduplication and compression to keep the backup size low. In this case, a tool like ``borg`` will be more appropriate.