adhere to the docker linter for the haskell image

dodona-haskell.dockerfile

@@ -1,34 +1,34 @@
 FROM haskell:8.2
 
-# Install jq for json querying in bash
-RUN ["apt-get", "update"]
-RUN ["apt-get", "-y", "install", "jshon"]
-RUN ["apt-get", "-y", "install", "freeglut3-dev"]
-
-# Make sure the students can't find our secret path, which is mounted in
-# /mnt with a secure random name.
-RUN ["chmod", "711", "/mnt"]
-
-# Add the user which will run the student's code and the judge.
-RUN ["useradd", "-m", "runner"]
+RUN apt-get update \
+ # Install jq for json querying in bash
+ # Install freeglut headers for gloss compilation
+ && apt-get install -y --no-install-recommends \
+        jshon=20131010-3+b1 \
+        freeglut3-dev=2.8.1-3 \
+ && rm -rf /var/lib/apt/lists/* \
+ # Make sure the students can't find our secret path, which is mounted in
+ # /mnt with a secure random name.
+ && chmod 711 /mnt \
+ # Add the user which will run the student's code and the judge.
+ && useradd -m runner
 
 # As the runner user
 WORKDIR /home/runner
 USER runner
-
-    # Install the cabal packages
-    RUN ["cabal", "update"]
-    RUN ["cabal", "install", "happy-1.19.9"]
-    RUN ["cabal", "install", "hlint-2.1.10"]
-    RUN ["cabal", "install", "QuickCheck-2.10.1"]
-    RUN ["cabal", "install", "HUnit-1.6.0.0"]
-    RUN ["cabal", "install", "MissingH-1.4.0.1"]
-    RUN ["cabal", "install", "json-builder-0.3"]
-    RUN ["cabal", "install", "stm-2.4.5.0"]
-    RUN ["cabal", "install", "gloss-1.13.0.1"]
-
-    # Create the working directory
-    RUN ["mkdir", "workdir"]
+RUN cabal update \
+ # happy must be installed to install haskell-src-exts
+ && cabal install happy-1.19.9 \
+ && cabal install \
+        hlint-2.1.10 \
+        QuickCheck-2.10.1 \
+        HUnit-1.6.0.0 \
+        MissingH-1.4.0.1 \
+        json-builder-0.3 \
+        stm-2.4.5.0 \
+        gloss-1.13.0.1 \
+ # Create the working directory
+ && mkdir workdir
 
 USER root