[mahdi2ba]

Fixes #340.

Change

• themes/treetracker/login/register.ftl: Keycloak's base register.ftl plus the socialProviders section copied from login.ftl. The base file is the 22.0.4 one on purpose: prod runs Keycloak 22.0.4 and dev 23.0.3, and the 23 template adds a "show password" button that does not exist on 22. The 22 template is valid on both, so dev and prod ship the same file.
• Dockerfile: KC_VERSION build argument instead of a hard-coded version (dev 23.0.3, prod 22.0.4 were in commented lines before). Also fixes the FROM/AS casing warning.
• Image tags: mahdibenayed1/keycloak-23:1.8 (dev, deployed) and mahdibenayed1/keycloak:1.8 (prod, built and pushed, not deployed yet).

Tested on dev

• Registration page shows the form, the Register button, then the Google and GitHub buttons; both link to the broker login of the session. Login page unchanged. No errors in the Keycloak log.

Prod deploy after merge: kubectl set image on treetracker-keycloak (1 replica, about one minute without login), then the same checks.