If you're using readeck as a container behind a reverse proxy, you must set the environment variable READECK_USE_X_FORWARDED=1.
Furthermore, you must adjust the allowed_hosts directive in the Readeck config at config.toml. The configuration file is stored within the Docker bind volume mount. Please define your subdomain(s) in use as follows:
[server]
host = "0.0.0.0"
port = 8000
allowed_hosts = ["readeck.example.com"]
use_x_forwarded_for = true
use_x_forwarded_host = true
use_x_forwarded_proto = true