Another proof for how stupid I am about Docker. Yep, it’s obviously not a docker-compose file. Is it called “Docker file”? Or does that term refer to the config.json? Whatever, your advice pointed me to the right direction, just the hook should be called before_code instead of before_web.
In short: Set up a socks5 with shadowsocks-libev, listen on the local machine on 172.17.0.1 (not localhost), pass on the proxy info as in your message, rebuild app.
I will write an in-depth guide here, as I assume there are more people out there going through this painful experience. Right now I am still facing issues with theme component repositories, so my rebuild has not succeeded yet, but I have passed at least all the plugin fetches.
Slightly irrelevant to the very topic, but the pain I am facing is not to be able to start the app, as the existing redis-server config - which we have on another machine - does not match the reality of the current app status. So I can’t start the container and deactivate the theme components via the GUI, which face a timeout when cloning them.