ci: update ci docker readme

Signed-off-by: Noah Watkins <[email protected]>

docker/ci/README.md

@@ -1,9 +1,22 @@
 zlog-ci
 =======
 
-zlog continuous integration docker image.
+This Dockerfile is used to create an image runs the zlog continuous
+integration test suite.
 
 Usage
 -----
 
-`docker run`
+By default the image will checkout and build zlog from the master branch of
+the repository located at https://github.com/noahdesu/zlog:
+
+  `docker run zlog/ci`
+
+Use `-e "branch=different-branch` to checkout a different branch:
+
+  `docker run -e "branch=testing" zlog/ci`
+
+A zlog source directory on the host can be used instead using a bind mount. In
+this case the `branch` environment variable is ignored:
+
+  `docker run -v /tmp/host_zlog:/src/zlog zlog/ci`