RHL-005-feat(docker): add .dockerignore to exclude env files and build outputs

.dockerignore

@@ -0,0 +1,14 @@
+# env files (never bake secrets into images)
+.env*
+!.env*.example
+
+# dependencies / build output
+node_modules
+.next
+coverage
+.git
+.DS_Store
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+.pnpm-debug.log*