fix(deploy): switch from nixpacks to Dockerfile + change branch to main

Nixpacks auto-detect could not find a 'start' script in package.json
and bailed out. Astro builds to static files in dist/ — there is no
Node server to start. Switching to a Dockerfile + nginx fixes the
'No start command could be found' error from EasyPanel.

The workflow also pointed at the source-code branch, but the panel's
git source ref for the dealplustech-astro service is 'main', so the
trigger was firing for the wrong ref. Both workflows now run on push
to main.

- Dockerfile: multi-stage node:20-alpine build + nginx:1.27-alpine serve
- nginx.conf: gzip, security headers, 1-year cache for hashed assets,
  try_files fallback for UTF-8 slugs (Astro file-based routing)
- .dockerignore: keep build context small (skip CI, docs, .gitea, IDE)
- build-and-deploy.yml + lint.yml: branch source-code -> main
- docs/ci-setup.md: corrected project + service names (customerwebsite
  / dealplustech-astro), documented the Dockerfile rationale, added a
  note for the 'Failed to sync changes' server-side error

.dockerignore

@@ -0,0 +1,34 @@
+# Build artifacts
+node_modules/
+dist/
+.astro/
+
+# CI / Gitea
+.gitea/
+.github/
+
+# Local dev / IDE
+.DS_Store
+.vscode/
+.idea/
+*.log
+*.tmp
+.env
+.env.*
+!.env.example
+
+# Git
+.git/
+.gitignore
+
+# Docker (don't include Docker files in the build context of themselves)
+Dockerfile
+.dockerignore
+docker-compose*.yml
+
+# Docs
+docs/
+
+# Misc
+coverage/
+*.tsbuildinfo