Allow using symlink for custom static files

[iromli]

Since upgrading from Jetty 10.0.6 (Gluu 4.4.0) to newer version, symlinks under oxauth/custom/static directory are no longer accessible via /oxauth/ext/resources/$filename URL.

As per Jetty 10.0.7, symlinks are not accessible by default. See the following links for workaround:

• https://www.eclipse.org/lists/jetty-users/msg10049.html
• https://www.eclipse.org/lists/jetty-users/msg10051.html

Related issue: https://github.com/GluuFederation/docker-oxauth/issues/57