Fix build command

Author: lazamar

build/Dockerfile

@@ -44,7 +44,7 @@ RUN mv /tmp/emulator /opt/emulator
 WORKDIR /opt/emulator
 RUN git fetch # just in case we have cached git clone
 RUN git checkout v1.9
-RUN cabal build emulator
+RUN cabal build exe:emulator
 RUN cp $(cabal list-bin emulator) /tmp/emulator
 
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