Add Docker support: Arch/Apache/PHP image, three volumes, docs

Adds a root Dockerfile (Arch Linux + Apache + PHP) and docker-compose.yml
with separate cache/data/images volumes, so a project's own content,
page cache, and future uploaded images stay out of paths a framework
update would wipe. App/ is baked into the image but can be bind-mounted
to override without a rebuild. Renames the Sass build-tool Dockerfile
example in the styling doc to Dockerfile.sass to avoid colliding with
the new app Dockerfile, adds a new /admin/docs/docker page (linked from
the docs index and nav), and documents the reserved images/ directory
in AGENTS.md and README.

Also records the user's git/docker execution permission boundaries in
CLAUDE.md for future sessions.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitignore

@@ -7,3 +7,5 @@
 /data/*.sqlite-shm
 .claude/
 /graphify-out/
+/images/*
+!/images/.gitkeep