Allow mounting a host volume as read-only

Drone currently supports mounting volumes into CI pipelines from the host machine, but there doesn’t currently appear to be a way to mount said volumes as read-only or anything similar.

Adding support for such would make it a lot safer when mounting things such as /usr/bin/ or /etc/, such as when wanting to use installed binaries or config files from the host sytem.

we do have support in the codebase for the :ro suffix when defining global volumes:

however, this depends on the following pull request being merged: