diff --git a/docker/ghc-alpine.Dockerfile b/docker/ghc-alpine.Dockerfile
index a884169..8a4e84d 100644
--- a/docker/ghc-alpine.Dockerfile
+++ b/docker/ghc-alpine.Dockerfile
@@ -15,4 +15,4 @@ RUN git clone -b ghc-"$GHC_VERSION"-release https://gitlab.haskell.org/ghc/ghc.g
 
 COPY ../scripts/ghc-alpine-build.sh /ghc-alpine-build.sh
 RUN chmod +x /ghc-alpine-build.sh
-RUN ./ghc-alpine-build.sh
+ENTRYPOINT ["/ghc-alpine-build.sh"]
diff --git a/scripts/ghc-alpine-build.sh b/scripts/ghc-alpine-build.sh
index 8e200fd..6f8247f 100644
--- a/scripts/ghc-alpine-build.sh
+++ b/scripts/ghc-alpine-build.sh
@@ -5,7 +5,7 @@ set -ex
 . "$HOME/.ghcup/env"
 
 cd ghc/
-cabal install happy alex --install-method=copy --constraint 'happy < 1.20'
+cabal install happy-1.20 alex --install-method=copy
 cp ~/.cabal/bin/* /usr/bin
 
 # Build GHC