Add .dockerignore and fix stale build cache

.dockerignore prevents node_modules and dist from leaking into the
build context, which could silently overwrite npm-installed packages.
Forced no-cache rebuild confirms dotenv and all deps install cleanly.

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

.dockerignore

@@ -0,0 +1,10 @@
+node_modules
+server/node_modules
+client/node_modules
+client/dist
+.env
+.git
+.gitignore
+*.md
+.todos/
+.sidecar/