Simplify math expressions

Dockerfile

@@ -100,7 +100,7 @@ RUN \
                     fileName="screens/install-try$try-$x.png"; \
                     import -window root "$fileName"; \
                     tiv -w 10000 -h 10000 "$fileName"; \
-                    x=$(bash -c "declare -i x=$x+1; echo "'$x'); \
+                    x="$(expr "$x" + 1)"; \
                     echo $x; \
                     sleep 1; \
                 done; \
@@ -151,7 +151,7 @@ RUN \
     do \
         echo "Restarting Installation"; \
         # Increase counter
-        y=$(bash -c "declare -i y=$y+1; echo "'$y'); \
+        y="$(expr "$y" + 1)"; \
     done && \
     kill -9 "$displayPID" > /dev/null 2>&1; \
     # Delete unfinished installations