Fix version check

Author: beriberikix

arduino/Dockerfile

@@ -21,6 +21,6 @@ RUN \
 
 RUN \
   if [ "$URL" != "" ] ; then export URL_FINAL="--additional-urls ${URL}" ; else export URL_FINAL="" ; fi \
-  && if [ "$VERSION" != "" ] ; then export CORE_FINAL="${CORE}@${VERSION}" ; else export CORE_FINAL="${CORE}" ; fi \
+  && if [ "$VERSION" != "latest" ] ; then export CORE_FINAL="${CORE}@${VERSION}" ; else export CORE_FINAL="${CORE}" ; fi \
   && arduino-cli core update-index ${URL_FINAL} \
   && arduino-cli core install ${CORE_FINAL} ${URL_FINAL}