Update Keycloak instructions

[froque]

No description provided.

[kkimurak]

Context: Keycloak instructions and example docker-compose-keycloak.yml are introduced with PR #2070, merged on 2020-3-12 and never been updated.

This PR update keycloak used in example:

• from: 8.0.1 (image last updated on 2020-1-28, git tag created on 2019-12-2)
• to: 26.4 (is currently 26.4.7-0, image last updated on 2025-12-1, github release created on 2025-12-1)

As previous image jboss/keycloak is not available on docker hub anymore, it now uses quay.io/keycloak/keycloak.

Current documentation (start-keycloak-container.adoc on keycloak/keycloak@ed69f65a9c, 2025-9-6) and its initial version (start-keycloak-container.adoc on keycloak/keycloak-web@702e368, 2020-3-20) referes quay.io so I'm not sure when it has done.

In keycloak 17.0 and later, some authentication endpoints are not prefixed with /auth by default (need explicit startup option like --http-relative-path /auth).
Also, environment variables for keycloak image is now prefixed with KC_.
This PR updates corresponding configuration parameters in docker-compose-keycloak.yml.

---

note: I'm not keycloak expert so not sure, but v17.0 may be the first release of Quarkus (Java framework) based version that is currently published as keycloak/keycloak. The change about prefix may be related to framework migration from WildFly (previously known as JBoss Application Server) to Quarkus. WildFly-based keycloak (-legacy or jboss/keycloak) have been reached to end of support on 2022-6.
As far as I checked commit history roughly, I couldn't immediately find out whether the quarkus version didn't adopt the /auth prefix from the beginning, or whether it was deleted in some commit. .. Well, that's not relevant to what will be done in this pull request.

For more details, refer https://www.keycloak.org/migration/migrating-to-quarkus