Enable configuration of of the 'extension-priority' parameter via environment variable

The 'extension-priority' configuration option is very useful for allowing multiple auth components to work together without one forcibly overriding the other. This is described multiple places in the documention (example: https://guacamole.apache.org/doc/gug/openid-auth.html under section 'Presenting unauthenticated users with a login screen').

This change to the docker image entrypoint script would allow this option to be set via environment variable in the docker container configuration. Currently, this is not supported by the entrypoint script, but based on the documentation (https://guacamole.apache.org/doc/gug/configuring-guacamole.html) it is implied that it should.

This should correct that, for this option.

An existing pull request exists to effect this change:  https://github.com/apache/guacamole-client/pull/686