feat: Add .dockerignore and configure Dockerfile for Flutter web build with updated base image.

.dockerignore

@@ -0,0 +1,21 @@
+# Git
+.git
+.gitignore
+
+# Flutter/Dart
+.dart_tool/
+.flutter-plugins
+.flutter-plugins-dependencies
+.packages
+.pub-cache/
+.pub/
+build/
+
+# IDEs
+.idea/
+.vscode/
+*.iml
+
+# OS
+.DS_Store
+Thumbs.db