docs(example): add components and diagram to docker compose

examples/docker-compose/README.md

@@ -19,9 +19,43 @@ make stop
 ```
 
 When the containers are up, browse to [http://ui-app.localhost](http://ui-app.localhost). There are a few controls:
-- Login: redirects to the IdP (Keycloak). Login with `alice`:`password`.
-- Call API: sends a GET request to http://api.localhost/json, which
+- **Login**: redirects to the IdP (Keycloak). Login with `alice`:`password`.
+- **Call API**: sends a GET request to http://api.localhost/json, which
     requires authentication. If the UI app has a token, it will be used.
-- Logout: call JWT block, blocking the token from making future
+- **Logout**: call JWT block, blocking the token from making future
     authenticated calls.
-- Clear State: clears the tokens from the UI app.
+- **Clear State**: clears the tokens from the UI app.
+
+## Components
+
+There are four small hosts and a proxy in this example.
+1. `http://ui-app.localhost`
+    - A static web UI loaded into the browser with demo controls.
+    - This app has the OIDC login flow callback.
+    - Logout will call the JWT Block service to add the access token to the blocklist.
+1. `http://keycloak.localhost`
+    - A Keycloak instance used as an OIDC identity provider.
+    - The web app will redirect here for user login.
+    - The demo user is `alice`:`password`.
+1. `http://api.localhost`
+    - A protected httpbin instance web service behind authentication.
+    - This requires a valid access token that is not in the blocklist.
+1. `http://jwtblock.localhost`
+    - A JWT Block instance.
+    - Nginx will defer to this service for authentication decisions (`auth_request` directive).
+    - If the JWT is invalid or is in the blocklist then it returns HTTP 401.
+    - This also has the "logout" endpoint, where an access token is added to the blocklist.
+
+```mermaid
+graph TD
+    WebUI["Web UI \n(html/js)"]
+    WebProxy["Web Proxy \n(nginx)"]
+    AuthProxy["Auth Proxy \n(jwtblock)"]
+    APIService["Web API \n(httpbin)"]
+    IdentityProvider["OpenID IdP \n(keycloak)"]
+
+    WebUI --> WebProxy
+    WebProxy --> IdentityProvider
+    WebProxy --> AuthProxy
+    WebProxy --> APIService
+```