feat(CUB-65): add multi-stage Dockerfile and .dockerignore for backend

backend/.dockerignore

@@ -0,0 +1,27 @@
+# Build artifacts
+bin/
+obj/
+
+# IDE / editor
+.vs/
+.vscode/
+*.user
+*.suo
+.idea/
+
+# Environment & secrets
+appsettings.Development.json
+.env
+.env.*
+
+# Docker
+Dockerfile
+.dockerignore
+
+# OS
+.DS_Store
+Thumbs.db
+
+# Misc
+*.md
+*.log

backend/Dockerfile

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+# ── Stage 1: Build ──────────────────────────────────────────
+FROM mcr.microsoft.com/dotnet/sdk:9.0 AS build
+WORKDIR /src
+
+# Copy csproj first for layer caching — restores before copying source
+COPY Extrudex.csproj .
+RUN dotnet restore
+
+# Copy the rest of the source
+COPY . .
+RUN dotnet publish Extrudex.csproj \
+    -c Release \
+    -o /app/publish \
+    --no-restore
+
+# ── Stage 2: Runtime ────────────────────────────────────────
+FROM mcr.microsoft.com/dotnet/aspnet:9.0 AS runtime
+WORKDIR /app
+
+# Non-root user for security
+RUN adduser --disabled-password --gecos "" appuser
+USER appuser
+
+# Copy published output from build stage
+COPY --from=build /app/publish .
+
+# ASP.NET Core listens on 8080 by default in .NET 8+
+EXPOSE 8080
+
+ENTRYPOINT ["dotnet", "Extrudex.dll"]