Windup-web-distribution: Document/Emphasize the setting of correct system property for keycloak properties

There is https://github.com/windup/windup-web/blob/master/KEYCLOAK-SETUP.md
described system properties on using the windup-web docker image and we need to emphasize to use system property on running docker container
and overwrite default value (localhost:8080/auth) in

windup-web-distribution/standalone/configuration/standalone-full.xml

for system property

keycloak.server.url