Fix correct elm compiler on PATH

Author: vic

elmer-json-compiler/Dockerfile

@@ -7,6 +7,7 @@ RUN  curl https://s3.amazonaws.com/rebar3/rebar3 > /bin/rebar3
 RUN  chmod 755 /bin/rebar3
 
 RUN  git clone --depth 1 https://github.com/elmer-compiler/elm-platform.git
-RUN  cd elm-platform/installers && runhaskell BuildFromSource.hs master
+RUN  cabal update
+RUN  cd /elm-platform/installers && runhaskell BuildFromSource.hs master
 
-ENV  PATH $PWD/elm-platform/installers/master/.cabal-sandbox/bin:$PATH
+ENV PATH /elm-platform/installers/Elm-Platform/master/.cabal-sandbox/bin:$PATH