add documentation

docs/content/doc/setup/config.md

@@ -27,6 +27,31 @@ first:
     child: true
 {{< /highlight >}}
 
+## Docker
+Environment variables that contain senstitive data can be suffixed with `_FILE`. If you take the expected environment variable for the configuration 
+option with the _FILE suffix at the end, the contents of the file it points to will be used. See [Docker Swarm](https://docs.docker.com/engine/swarm/secrets/) 
+and [Docker Compose](https://docs.docker.com/compose/use-secrets/) docs for more info on using secrets.
+
+{{< highlight bash >}}
+echo "a super secure random secret" > /host/secrets/path/vikunja_service_jwtsecret
+{{< /highlight >}}
+
+{{< highlight yaml >}}
+version: '3'
+
+secrets:
+  vikunja_service_jwtsecret:
+    file: /host/secrets/path/vikunja_service_jwtsecret
+
+services:
+  api:
+    image: vikunja/api
+    secrets:
+      - vikunja_service_jwtsecret
+    environment:
+      VIKUNJA_SERVICE_JWTSECRET_FILE: /run/secrets/vikunja_service_jwtsecret
+{{< /highlight >}}
+
 # Formats
 
 Vikunja supports using `toml`, `yaml`, `hcl`, `ini`, `json`, envfile, env variables and Java Properties files.