chore: add .dockerignore

Defensive — keeps node_modules/dist/.vite/.env out of the build context
when running `docker build` locally. Portainer's Stacks→Repository flow
clones from git (which already excludes these via .gitignore), so this
doesn't change Portainer behavior, just hardens local-docker-build paths.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.dockerignore

@@ -0,0 +1,18 @@
+.git
+.gitignore
+node_modules
+**/node_modules
+dist
+**/dist
+.vite
+**/.vite
+.env
+.env.*
+!.env.example
+*.log
+coverage
+.pnpm-store
+*.tsbuildinfo
+**/*.tsbuildinfo
+.DS_Store
+README.md