Add link to docker build repo

- Created new repo to run from docker/alpine
  (see [ctags-docker](https://github.com/universal-ctags/ctags-docker))
- Link in README

README.md

@@ -25,6 +25,11 @@ Go to the [releases](https://github.com/universal-ctags/ctags-win32/releases) pa
 ### Mac
 See [Homebrew Tap for Universal Ctags](https://github.com/universal-ctags/homebrew-universal-ctags)
 
+### Docker
+Go to [ctags-docker](https://github.com/universal-ctags/ctags-docker) and follow
+instructions to download the `uctags` script. Docker will automatically fetch
+the image from Docker Hub.
+
 ## How to build and install ##
 
 To build with Autotools, see `docs/autotools.rst` for more information.