Selfhosted

The user and group id inside the container doesn't have to match any user on your host machine. It's possible that user:70 is configured as the user to launch inside the container, in which case you should set the ownership of the directory to match what the container expects.

Eg: The container for my torrent client runs as user 700 group 700. My host machine does not have either of those IDs defined. My torrent directory must be chown'd by 700:700 or else the container can't read/write torrents.

I checked out your edit and what's going on is pretty weird. my recommendation (if you don't wanna just use the native import/export feature and give up on DB dumps) is to (on the new server) create a minimal tandoor docker-compose.yml, run it, do minimal setup, see if it works, and if the fresh container works, check the permissions, then just replicate those perms for your other postgres directory that contains data you actually care about. good luck!

In response to your update: Try specifying the user that's supposed to own the mapped directories in the docker compose file. Then make sure the UID and GID you use match an existing user on the new system you are testing the backup on.

First you need to get the id of the user you want to run the container as. For a user called foo, run id foo. Note down the UID and GID.

Then in your compose file, modify the db_recipes service definition and set the UID and GID of the user that should own the mapped volumes:

  db_recipes:
    restart: always
    image: postgres:15-alpine
    user: "1000:1000" #Replace this with the corresponding UID and GID of your user
    volumes:
      - ./postgresql:/var/lib/postgresql/data
    env_file:
      - ./.env

Recreate the container using docker compose up -d (don't just restart it; you need to load the new config from the docker compose file). Then inspect the postgresql directory using ls -l to check whether it's actually owned by user with UID 1000 and group with GID 1000. This should solve the issue you are having with that backup program you're using. It's probably unable to copy that particular directory because it's owned by root:root and you're not running it as root (don't do that; it would circumvent the real problem rather than help you address it).

Now, when it comes to copying this to another machine, as already mentioned you could use something that preserves permissions like rsync, but for learning purposes I'd intentionally do it manually as you did before to potentially mess things up. On the new machine, repeat this process. First find the UID and GID of the current non-root user (or whatever user you want to run your containers as). Then make sure that UID and GID are set in the compose files. Then inspect the directories to make sure they have the correct ownership. If the compose file isn't honoring the user flag or if the ownership doesn't match the UID and GID you set for whatever reason, you can also use chown -R UID:GID ./postgresql to change ownership (replace UID:GID with the actual IDs), but that might get overwritten if you don't properly specify it in the compose file as well, so only do it for testing purposes.

Edit: I also highly recommend using CLIs (terminal) instead of the GUI for this sort of thing. In my experience, the GUIs aren't always designed to give you all the information you need and can actually make things more difficult for you.