docker/keycloak: Fix custom swh theme installation for Keycloak 12

Since Keycloak 12, it is no longer possible to use a custom theme from a directory symbolic link, so make a raw copy of the swh theme in the Keycloak theme folder instead.

For the record, I opened an issue on the subject in Keycloak JIRA a couple of weeks ago and I got the following answer:

Rejecting this as a bug, as we want to make sure only files within the given 
directory can be used for security reasons. We never intended to support symlinks, 
which was a side-effect of a bug/vulnerability that was resolved.

---

Migrated from D5119 (view on Phabricator)

docker/services/keycloak/entrypoint.sh

@@ -7,7 +7,7 @@ cd
 if [ ! -d swh-keycloak-theme ]
 then
   git clone https://forge.softwareheritage.org/source/swh-keycloak-theme.git
-  ln -s /opt/jboss/swh-keycloak-theme/swh/ /opt/jboss/keycloak/themes/swh
+  cp -r /opt/jboss/swh-keycloak-theme/swh /opt/jboss/keycloak/themes/swh
 fi
 
 echo "Starting Keycloak"