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.txt

@@ -0,0 +1,15 @@
+admin
+email
+error
+flex-objects
+form
+login
+problems
+sitemap
+social-meta-tags
+draft-preview
+langswitcher
+automagic-images
+admin-media-move
+admin-media-replace
+admin-media-actions