wallabag: Wallabagger for Firefox not working with self-hosted instance (Docker)

Issue details

I just deployed a Wallabag instance to my server using the official docker-compose. I also use Nginx as a reverse proxy.

Docker compose and Nginx configs here.

The web UI and the Android client are working fine but the Wallabagger for Firefox extension does not work. I configured it properly (and successfully) and when I try to bag a page I get the “Saving the page to wallabag…” message forever. The Wallabag icon turns yellow upon clicking and then red. See screenshot bellow.

Looking at the docker-compose logs (when running up without the `-d’ option) shows me this error when I try to save a page with Wallabagger:

db_1        | 2018-09-18 12:17:26 58 [Warning] Aborted connection 58 to db: 'wallabag' user: 'wallabag' host: '172.23.0.3' (Got an error reading communication packets)
wallabag_1  | 2018/09/18 12:17:26 [error] 245#245: *164 recv() failed (104: Connection reset by peer) while reading response header from upstream, client: 172.23.0.1, server: _, request: "POST /api/entries.json HTTP/1.0", upstream: "fastcgi://127.0.0.1:9000", host: "127.0.0.1:3004"
wallabag_1  | 172.23.0.1 - - [18/Sep/2018:12:17:26 +0000] "POST /api/entries.json HTTP/1.0" 502 166 "-" "Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:62.0) Gecko/20100101 Firefox/62.0"

Environment

• Version: 2.3.3
• Install method: Docker (docker-compose)
• Last wallabag version that did not exhibit the issue (if applicable): -
• PHP version: 7.2.10 (Docker version, should be irrelevant)
• OS: Debian 8
• Type of hosting: dedicated
• Storage system: MariaDB

Steps to reproduce/test case

1. Deploy Wallbag with docker-compose and a Nginx reverse proxy as mentioned above
2. Configure the Wallabagger extension for Firefox
3. Try to save a page with it

Screenshot