Minor fixes to Dockerfile

First of all, thanks for putting this together.  A lot of the gitolite images on the Docker registry broke about a month ago for some unknown reason.  I found your repo on Github, tried building it, and found these 2 minor problems.  Fixed them, and my gitolite image now works great.  Thanks!

* There was a trailing backslash on one line that was preventing the next line from running correctly.
* There was a permissions issue after admin.pub was added to the image.  It was owned by image root, and thus unreadable by the git user.