make it possible to use read-only configuration again in Docker

[vladak]

As mentioned in #3522 (comment) it should be possible to pass read-only configuration to Docker. This got broken with the transition to the sync.py. It should still be possible to use it when projects are disabled.

[Teriand]

I use this command to use RO config:
OG_DOCKER="docker exec opengrok opengrok-indexer -C -a /opengrok/lib/opengrok.jar -- -c /usr/local/bin/ctags -s /opengrok/src/ -d /opengrok/data -H -P -S -G -W /opengrok/etc/configuration.xml -U http://localhost:8080/ -R /opengrok/etc/readonly_configuration.xml"