Move plugins to manifest, pin Docker version, add Makefile

- Add plugins.txt listing all plugins for reproducible installs
- Add Makefile with setup/start/stop/install-plugins targets
- Remove user/plugins/ from git tracking
- Pin Docker image to 1.7.49.5-ls244

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

plugins/sitemap/templates/sitemap-extensions/image.xml.twig

@@ -1,17 +0,0 @@
-<image:image>
-{% if image.loc %}
-  <image:loc>{{ url(image.loc, true) }}</image:loc>
-{% endif %}
-{% if image.caption %}
-  <image:caption>{{ image.caption }}</image:caption>
-{% endif %}
-{% if image.geoloc %}
-  <image:geo_location>{{ image.geoloc }}</image:geo_location>
-{% endif %}
-{% if image.title %}
-  <image:title>{{ image.title }}</image:title>
-{% endif %}
-{% if image.license %}
-  <image:license>{{ image.license }}</image:license>
-{% endif %}
-</image:image>