Merge pull request #13 from dodona-edu/fix/r-lint-fixes

Fix lint errors in the R dockerfile

dodona-r.dockerfile

@@ -1,9 +1,11 @@
-FROM r-base
+FROM r-base:3.6.1
 
 # Make sure the students can't find our secret path, which is mounted in
 # /mnt with a secure random name.
 RUN apt-get update && \
-  apt-get install -y --no-install-recommends procps && \
+  apt-get install -y --no-install-recommends procps=3.3.15 && \
+  apt-get clean && \
+  rm -rf /var/lib/apt/lists/* && \
   chmod 711 /mnt && \
   groupmod -n runner docker && \
   usermod -l runner -d /home/runner docker && \
@@ -15,3 +17,4 @@ RUN apt-get update && \
 
 WORKDIR /home/runner/workdir
 COPY main.sh /main.sh
+