Fix paths after update of checkout action

Fix paths after update of checkout action

The Github 'checkout' action was updated from v1 to v3. But the behaviour
changed between the two which broke this workflow.
The old action would clone other repositories not into the workspace
but parallel to the workspace. The new version does every clone/checkout
relative to the workspace. That means that where previously the
gitblit-docker repository would be cloned in parallel to the workspace
which is gitblit/gitblit, it is now cloned into the gitblit/gitblit
directory path.

So remove all the references to `../gitblit-docker`. The files are in
the current directory now.