Model the build on mozimo and target emily

- Makefile: PLATFORMS, tags :<commit> and :main, build-check with --pull.
- deploy.sh: push, guards, git pull --ff-only, optional --prune, image_tag.
- Replace .containerignore with .dockerignore.
- Add the opt-in image prune to the hops role.
- Tag the Caddyfile task so --skip-tags caddy works before DNS cutover.
- Trim comments and rewrite build and deploy text in plain prose.

.dockerignore

@@ -0,0 +1,39 @@
+# Dependencies. Installed inside the image.
+node_modules
+**/node_modules
+
+# Build output. Produced by the builder stage.
+frontend/.output
+frontend/.tanstack
+.output
+dist
+
+# Version control and editor.
+.git
+.gitignore
+.dockerignore
+.vscode
+.idea
+.agents
+.gemini
+
+# Environment and local state.
+.env
+.env.*
+backend/pb_data
+backend/pocketbase
+backend/pocketbase.exe
+*.db
+*.db-shm
+*.db-wal
+
+# Caches and logs.
+*.log
+npm-debug.log*
+scratch
+page.html
+
+# Repo tooling not needed inside the image.
+ansible
+Makefile
+README.md