Add Docker support with multi-stage build

Enable output: standalone in next.config.ts so Next.js produces a
self-contained server bundle in .next/standalone. Multi-stage Dockerfile
(deps -> builder -> runner) on node:20-alpine keeps the final image
minimal; app runs as a non-root user for security. .dockerignore
excludes node_modules, .next, .env files, and editor tooling.

To build and run:
  docker build -t portfolio .
  docker run -p 3000:3000 portfolio

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.dockerignore

@@ -0,0 +1,22 @@
+# Build output & deps — recreated inside the image
+.next
+node_modules
+
+# Source control
+.git
+.gitignore
+
+# Local env files — never bake secrets into an image
+.env
+.env.*
+!.env.example
+
+# Editor / tooling
+.claude
+.vscode
+*.log
+npm-debug.log*
+
+# OS noise
+.DS_Store
+Thumbs.db