added link to docker image

Author: bwbush

ReadMe.md

@@ -40,6 +40,8 @@ The executable result will be in `./build/bin/mantis`.
 
 Alternatively, one can use the `cabal install` installation approach, which relies on the [cabal.project](cabal.project) file and which is known to succeed with cabal 3.4.0.0 and ghc 8.10.4.
 
+A docker image is available at https://github.com/wutzebaer/mantis-docker/.
+
 
 ### Development environment