.github/workflows/ci.yml: clean up Dockerfile GHC version matching check (#1258)

.github/workflows/ci.yml

@@ -102,15 +102,13 @@ jobs:
           stack exec -- ihaskell install --stack
           test/acceptance.nbconvert.sh stack exec -- jupyter nbconvert
       - name: Check Dockerfile GHC version matches
+        if: ${{ matrix.versions.stack-yaml == 'stack.yaml' }}
         run: |
-          if [[ '${{ matrix.versions.stack-yaml }}' != 'stack.yaml' ]]; then
-            exit 0
-          fi
-
           set -e
           STACK_GHC_VERSION=$(stack exec -- ghc --version | awk '{ print $NF }')
           DOCKER_GHC_VERSION=$(sed -n 's/ARG GHC_VERSION=\(.*\)/\1/p' Dockerfile)
           if [[ ${STACK_GHC_VERSION} != ${DOCKER_GHC_VERSION} ]]; then
             echo 'GHC_VERSION in Dockerfile does not match stack resolver'
+            echo "GHC_VERSION should be ${STACK_GHC_VERSION}"
             exit 1
           fi