Revert "Drop the host flag"

This reverts commit 8bdfe16249.

Going to need that to open up the network space outside of the
container. See https://github.com/moby/moby/issues/28121.

sbin/entrypoint.sh

@@ -8,6 +8,7 @@ run_jekyll() {
 
   bundle exec jekyll serve \
     --port "$JEKYLL_PORT" \
+    --host "$JEKYLL_HOST" \
     --trace
 }