Look here, I might have had the same issue: https://lemmy.ml/post/1509599

Yeah no, DNS works in the containers:

dot@deepthought:~# docker exec -it lemmy-lemmy-1 ping pictrs
PING pictrs (192.168.7.2): 56 data bytes
64 bytes from 192.168.7.2: seq=0 ttl=64 time=0.079 ms

However, your hunch did actually turn out to be correct in that there’s still a networking issue: I opened a port for pictrs and hit the API with postman, and noticed it would show up in the pictrs logs which were empty before. So I changed the url to the host’s IP and the open port, and now it works. :)

lemmy.hjson:

  pictrs: {
    url: "http://192.168.123.21:4808"
    api_key: "apikey"
  }

docker-compose.yaml:

pictrs:
...
  ports:
        - "4808:8080"

Well, butter my butt and call me a biscuit. In implementing this, I stumbled upon an issue: I had the url in pictrs_config, when it should have been pictrs.

Now at least the logs are showing an error that matches the toaster:

2023-06-25T13:09:27.753749368Z 2023-06-25T13:09:27.753609Z  WARN lemmy_server::root_span_builder: error decoding response body: expected value at line 1 column 1
2023-06-25T13:09:27.753809022Z reqwest::Error { kind: Decode, source: Error("expected value", line: 1, column: 1) }

I already have those containers in an external network, and changing the URL hasn’t fixed it.

I did.

dot@deepthought:~# ls -al /sharedfolders/apps/lemmy/volumes/pictrs/
total 12
drwxrwsrwx 3  991  991 4096 Jun 22 20:24 .
drwxrwsrwx 5 root root 4096 Jun 25 09:34 ..
drwxr-sr-x 3  991  991 4096 Jun 22 20:24 sled-repo

dot@deepthought:~# docker ps | grep pictrs
14bd3db41d50   asonix/pictrs:0.4.0-beta.19                            "/sbin/tini -- /usr/…"   4 hours ago    Up 4 hours            6669/tcp, 8080/tcp                                                                                    lemmy-pictrs-1

dot@deepthought:~# docker logs lemmy-pictrs-1
2023-06-25T07:34:38.379801Z  INFO actix_server::builder: starting 6 workers
2023-06-25T07:34:38.379855Z  INFO actix_server::server: Actix runtime found; starting in Actix runtime