improve startup docs

platform/linux/startup.sh

@@ -1,5 +1,13 @@
 #!/bin/sh
 
-# This file will have any docker pre-installations
+if [ ! -d /app/.github ] || [ ! -d /home/user/.cache ]; then
+    echo " "
+    echo "ERROR: Docker container was not started properly."
+    echo "       From the root of this repo, run the following command."
+    echo "       You may add any command to perform in the container at the end of this command."
+    echo " "
+    echo '  docker run --rm -it -v "$PWD:/app/" -v "$PWD/.docker_cache:/home/user/.cache" maplibre-native-image'
+    exit 1
+fi
 
 exec "$@"