Fix docker image for validate-8.4.4 (used GHC 8.6.5)

.docker/validate-8.4.4.dockerfile

-FROM    phadej/ghc:8.6.5-bionic
+FROM    phadej/ghc:8.4.4-bionic
 
 # Install cabal-plan
 RUN     mkdir -p /root/.cabal/bin && \
@@ -16,7 +16,7 @@ RUN     cabal v2-install happy --constraint 'happy ^>=1.19.12'
 
 # Install some other dependencies
 # Remove $HOME/.ghc so there aren't any environments
-RUN     cabal v2-install -w ghc-8.6.5 --lib \
+RUN     cabal v2-install -w ghc-8.4.4 --lib \
           aeson \
           async \
           base-compat \
@@ -48,4 +48,4 @@ RUN     cabal v2-install -w ghc-8.6.5 --lib \
 # Validate
 WORKDIR /build
 COPY    . /build
-RUN     sh ./validate.sh -w ghc-8.6.5 -v
+RUN     sh ./validate.sh -w ghc-8.4.4 -v