build(Dockerfile): introduce tester stage

docker/Dockerfile

-FROM pyscipopt:4.0.0
+FROM pyscipopt:4.0.0 AS optimizer
 
 USER root
 
@@ -8,3 +8,15 @@ RUN python -m pip install --upgrade \
         ilp-keyboard-layout-optimization
 
 USER user
+
+FROM optimizer as tester
+
+USER root
+
+RUN python -m pip install --upgrade \
+    pip \
+    -i https://test.pypi.org/simple/  \
+        ilp-keyboard-layout-optimization[test]
+
+USER user
+