fix: correct inspector package name in Dockerfiles and add .dockerignore

- Change @anthropic-ai/sdk-inspector to @sandbox-agent/inspector in all Dockerfiles
- Add .dockerignore to exclude target/, node_modules/, etc from Docker context

The wrong package name caused pnpm install --filter to match nothing, so the
inspector frontend was never built, resulting in binaries without the /ui/ endpoint.

.dockerignore

@@ -0,0 +1,26 @@
+# Build outputs
+target/
+dist/
+build/
+
+# Dependencies
+node_modules/
+
+# Cache
+.cache/
+.turbo/
+*.tsbuildinfo
+
+# Environment
+.env
+.env.*
+
+# IDE
+.idea/
+.vscode/
+
+# OS
+.DS_Store
+
+# Git
+.git/