Readme: Fix link to nightly docker image

The link to the nightly docker images points to the actual image layer,
which is removed when a new nightly is pushed.
Instead, we should point to the images page, filtering on the tag 'nightly'.

README.markdown

@@ -6,7 +6,7 @@ Gitblit is an open source, pure Java Git solution for managing, viewing, and ser
 More information about Gitblit can be found [here](http://gitblit.com).
 
 <a href='https://github.com/gitblit/gitblit/releases/latest' title='Download'><img alt='download badge' src='https://img.shields.io/badge/dynamic/json?color=9cf&label=Download&query=%24.name&url=https%3A%2F%2Fapi.github.com%2Frepos%2Fgitblit%2Fgitblit%2Freleases%2Flatest'></a>
-<a title='nightly docker' href='https://hub.docker.com/layers/gitblit/gitblit/nightly/images/sha256-4bb3771046b572ae75a17445f0702ce4a33a940b1086fd52e2a4bbe70900145b'><img alt='nightly docker badge' src='https://img.shields.io/badge/dynamic/json?color=2496ed&label=Nightly Docker&query=%24.tag_last_pushed&cacheSeconds=43200&url=https%3A%2F%2Fhub.docker.com%2Fv2%2Frepositories%2Fgitblit%2Fgitblit%2Ftags%2Fnightly'></a>
+<a title='nightly docker' href='https://hub.docker.com/r/gitblit/gitblit/tags?page=1&name=nightly'><img alt='nightly docker badge' src='https://img.shields.io/badge/dynamic/json?color=2496ed&label=Nightly Docker&query=%24.tag_last_pushed&cacheSeconds=43200&url=https%3A%2F%2Fhub.docker.com%2Fv2%2Frepositories%2Fgitblit%2Fgitblit%2Ftags%2Fnightly'></a>
 ![License](https://img.shields.io/github/license/gitblit/gitblit?color=%23f29024)
 
 License