Keycloak: add dynamic user id & change export path

• The previous export path pointing to '/tmp' in the container was causing problems when migrating to new versions due to permissions.
• Additionally remove obsolete version tag in docker-compose.yml.

keycloak/Dockerfile

 ARG KC_TAG
+ARG KC_USER
 
 FROM registry.access.redhat.com/ubi9 AS ubi-micro-build
 COPY ca-certificates/* /etc/pki/ca-trust/source/anchors/
@@ -7,3 +8,5 @@ RUN update-ca-trust
 
 FROM quay.io/keycloak/keycloak:${KC_TAG}
 COPY --from=ubi-micro-build /etc/pki /etc/pki
+
+USER ${KC_USER}