refactor(Dockerfile): fix double slashes in pathes

Dockerfile

 from openjdk:11-jdk-slim
 
 env MODOCOT_BASEDIR /modocot
-env MODOCOT_TESTDIR /$MODOCOT_BASEDIR/test
-env MODOCOT_SOURCEDIR /$MODOCOT_BASEDIR/src
-env MODOCOT_RESULTDIR /$MODOCOT_BASEDIR/result
-env MODOCOT_LIBSDIR /$MODOCOT_BASEDIR/libs
+env MODOCOT_TESTDIR $MODOCOT_BASEDIR/test
+env MODOCOT_SOURCEDIR $MODOCOT_BASEDIR/src
+env MODOCOT_RESULTDIR $MODOCOT_BASEDIR/result
+env MODOCOT_LIBSDIR $MODOCOT_BASEDIR/libs
 
 run mkdir -p $MODOCOT_TESTDIR \
     && mkdir $MODOCOT_SOURCEDIR \