Docker improvements and decode loading state

- Fix .dockerignore to exclude *.img.xz files (was 2.3GB context)
- Remove deprecated 'version' attribute from docker-compose.yml
- Increase container memory limits to 2GB/1GB (prevent OOM on DCT)
- Add loading spinner to decode button during form submission

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontends/web/templates/decode.html

@@ -518,5 +518,8 @@ advancedOptionsDec?.addEventListener('show.bs.collapse', () => {
 advancedOptionsDec?.addEventListener('hide.bs.collapse', () => {
     document.getElementById('advancedChevronDec')?.classList.replace('bi-chevron-up', 'bi-chevron-down');
 });
+
+// Loading state for decode button
+Stegasoo.initFormLoading('decodeForm', 'decodeBtn', 'Decoding...');
 </script>
 {% endblock %}