Set the server's public URL from KEEPORG_EXTERNAL_URL

The image passes neither -config nor -url, so a deployment had no way to
set external_url and the server built invite links, device-approval pages
and the OIDC redirect URI on http://localhost:8765. The variable overrides
the configuration file and -url overrides it. The deployment values set it
to https://keeporg.tiredsysadmin.cc.

Also document every environment variable the server reads, and the
standard proxy and CA variables it honours for OIDC.

Server version 0.8.0.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -122,6 +122,19 @@ can no longer merge. `KEEPORG_MASTER_KEY` takes the base64 key directly if
 your secret store injects variables instead of files. See
 [packaging/docker/compose.yaml](packaging/docker/compose.yaml).
 
+Server environment variables:
+
+| Variable | Meaning |
+| --- | --- |
+| `KEEPORG_MASTER_KEY` | the master key, base64; overrides the other two sources |
+| `KEEPORG_MASTER_KEY_FILE` | path to a file holding the master key (base64 or 32 raw bytes); never created, and if it is missing the server runs but cannot create or merge vaults |
+| `KEEPORG_EXTERNAL_URL` | the public base URL (`external_url`); `-url` overrides it |
+| `KEEPORG_ADMIN_PASSWORD` | the first `admin` password, read only on an empty server and by `-reset-admin` |
+
+Everything else is configured in `server.json` or by flag. The details,
+including the standard proxy and CA variables, are in
+[docs/HANDOVER.md](docs/HANDOVER.md#environment-variables).
+
 ## Documentation
 
 - [docs/HANDOVER.md](docs/HANDOVER.md) — architecture, the decisions behind