Prerequisites

All examples in this guide use the public image. If you’ve mirrored the repository for your own use (for example, to your Docker Hub namespace), update your commands to reference the mirrored image instead of the public one.

For example:

• Public image: dhi.io/<repository>:<tag>
• Mirrored image: <your-namespace>/dhi-<repository>:<tag>

For the examples, you must first use docker login dhi.io to authenticate to the registry to pull the images.

What's included in this .NET image

This Docker Hardened .NET image provides secure, production-ready variants for both development and runtime scenarios:

Runtime variants include:

• .NET runtime for running compiled applications
• Essential libraries and dependencies
• Optimized for minimal attack surface

SDK variants (tagged with -sdk) include:

• Complete .NET SDK for building applications
• Package managers and development tools
• Build tools and compilers
• Shell access for development workflows

Start a .NET application

Run the following command:

# Run a .NET application (runtime variant)
docker run --rm dhi.io/dotnet:<tag>

# Start an interactive development container (SDK variant)
docker run --rm -it --entrypoint bash dhi.io/dotnet:<tag>-sdk

To inspect the image configuration:

# Check entry point and user configuration
docker image inspect dhi.io/dotnet:8 --format='{{.Config.Entrypoint}}'
docker image inspect dhi.io/dotnet:8 --format='{{.Config.Cmd}}'
docker image inspect dhi.io/dotnet:8 --format='{{.Config.User}}'

# Compare image sizes
docker images | grep dotnet

Common .NET use cases

Build and run a .NET console application

Use a multi-stage Dockerfile to build your application with the SDK variant and run with the runtime variant:

# Build stage using SDK variant
FROM dhi.io/dotnet:8-sdk AS build
WORKDIR /src
COPY *.csproj .
RUN dotnet restore
COPY . .
RUN dotnet publish -c Release -o /app

# Runtime stage using minimal runtime variant
FROM dhi.io/dotnet:8
WORKDIR /app
COPY --from=build /app .
ENTRYPOINT ["dotnet", "YourApp.dll"]

Build and run commands:

docker build -t my-console-app .
docker run --rm my-console-app

ASP.NET Core web application

For web applications, use the ASP.NET Core runtime from the separate DHI ASP.NET Core repository:

# Build stage
FROM dhi.io/dotnet:8-sdk AS build
WORKDIR /src
COPY MyWebApp/*.csproj ./MyWebApp/
RUN dotnet restore MyWebApp/MyWebApp.csproj
COPY . .
RUN dotnet publish MyWebApp/MyWebApp.csproj -c Release -o /app

# Runtime stage for web apps
FROM dhi.io/aspnetcore:8
WORKDIR /app
COPY --from=build /app .
EXPOSE 8080
ENTRYPOINT ["dotnet", "MyWebApp.dll"]

Configure your ASP.NET Core application for nonroot user compatibility:

// In Program.cs, ensure proper port configuration
var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

app.MapGet("/", () => "Hello World from DHI .NET Web App!");
app.MapGet("/health", () => new { status = "healthy", timestamp = DateTime.UtcNow });

// Use port 8080 (nonroot user compatible)
app.Run("http://0.0.0.0:8080");

Build and run commands:

docker build -t my-web-app .
docker run -d --name my-web-app -p 8080:8080 my-web-app

# Test endpoints
curl http://localhost:8080/
curl http://localhost:8080/health

# Clean up
docker stop my-web-app && docker rm my-web-app

Development container with volume mounting

Mount your source code for live development:

# Start an interactive development environment
docker run --rm -it \
  -v $(pwd):/workspace \
  -w /workspace \
  dhi.io/dotnet:8-sdk \
  bash

Inside the container, you can run complete development workflows:

# Create a new project
dotnet new console -n MyApp
cd MyApp

# Add packages
dotnet add package Newtonsoft.Json

# Build and run
dotnet build
dotnet run

# Run tests
dotnet test

# Publish for production
dotnet publish -c Release -o ./publish

For quick development tasks without entering the container:

# Create a new project from outside the container
docker run --rm -v $(pwd):/workspace -w /workspace \
  dhi.io/dotnet:8-sdk \
  dotnet new web -n MyWebApp

# Build the project
docker run --rm -v $(pwd):/workspace -w /workspace \
  dhi.io/dotnet:8-sdk \
  dotnet build MyWebApp

# Create and run a simple console application
docker run --rm -v $(pwd):/workspace -w /workspace \
  dhi.io/dotnet:8-sdk \
  dotnet new console -n HelloWorld

docker run --rm -v $(pwd):/workspace -w /workspace \
  dhi.io/dotnet:8-sdk \
  dotnet run --project HelloWorld

Running pre-built applications

Execute .NET CLI commands to run pre-built applications:

# Create and build a console application using SDK
docker run --rm -v $(pwd):/workspace -w /workspace \
  dhi.io/dotnet:8-sdk \
  dotnet new console -n MyApp

docker run --rm -v $(pwd):/workspace -w /workspace \
  dhi.io/dotnet:8-sdk \
  dotnet build MyApp

# Run the built application using runtime image (note the full path to the compiled DLL)
docker run --rm -v $(pwd):/workspace -w /workspace \
  dhi.io/dotnet:8 \
  dotnet MyApp/bin/Debug/net8.0/MyApp.dll

Note: When using dotnet build, the compiled DLL is placed in bin/Debug/net8.0/ (or bin/Release/net8.0/ for release builds). When using dotnet publish in multi-stage builds, the DLL is copied directly to the specified output directory (/app in the examples above), which is why the multi-stage examples work with just the DLL name.

Kubernetes deployment

Deploy your .NET application to Kubernetes:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: dotnet-app
spec:
  replicas: 3
  selector:
    matchLabels:
      app: dotnet-app
  template:
    metadata:
      labels:
        app: dotnet-app
    spec:
      containers:
        - name: dotnet-app
          image: <your-namespace>/my-dotnet-app:latest
          ports:
            - containerPort: 8080
          env:
            - name: ASPNETCORE_ENVIRONMENT
              value: "Production"
            - name: ASPNETCORE_URLS
              value: "http://0.0.0.0:8080"
          livenessProbe:
            httpGet:
              path: /health
              port: 8080
            initialDelaySeconds: 30
            periodSeconds: 10
          readinessProbe:
            httpGet:
              path: /health
              port: 8080
            initialDelaySeconds: 5
            periodSeconds: 5
          resources:
            requests:
              memory: "128Mi"
              cpu: "100m"
            limits:
              memory: "256Mi"
              cpu: "500m"
---
apiVersion: v1
kind: Service
metadata:
  name: dotnet-app-service
spec:
  selector:
    app: dotnet-app
  ports:
    - port: 80
      targetPort: 8080
  type: LoadBalancer

Use Docker Compose

This is an example pattern for local development. This example cannot be run as-is and requires a complete application setup:

version: "3.8"
services:
  app:
    build: .
    ports:
      - "8080:8080"
    environment:
      - ASPNETCORE_ENVIRONMENT=Development
      - ASPNETCORE_URLS=http://0.0.0.0:8080
    volumes:
      - ./logs:/app/logs
    depends_on:
      - db

  db:
    image: postgres:15
    environment:
      POSTGRES_DB: myapp
      POSTGRES_USER: user
      POSTGRES_PASSWORD: password
    ports:
      - "5432:5432"
    volumes:
      - postgres_data:/var/lib/postgresql/data

volumes:
  postgres_data:

Note: This is provided as a reference pattern only. To use this, you would need:

• A working Dockerfile in your project root
• An ASP.NET Core application configured to connect to PostgreSQL
• Proper Entity Framework or database connection setup

Docker Official Images vs. Docker Hardened Images

Why no shell or package manager?

Docker Hardened Images prioritize security through minimalism:

• Reduced attack surface: Fewer binaries mean fewer potential vulnerabilities
• Immutable infrastructure: Runtime containers shouldn't be modified after deployment
• Compliance ready: Meets strict security requirements for regulated environments

The hardened images intended for runtime don't contain a shell nor any tools for debugging. Common debugging methods for applications built with Docker Hardened Images include:

• Docker Debug⁠ to attach to containers
• Docker's Image Mount feature to mount debugging tools
• Ecosystem-specific debugging approaches

Docker Debug provides a shell, common debugging tools, and lets you install other tools in an ephemeral, writable layer that only exists during the debugging session.

For example, you can use Docker Debug:

# Start your application
docker run -d --name my-app dhi.io/dotnet:8

# Attach debugger
docker debug my-app

or mount debugging tools with the Image Mount feature:

docker run --rm -it --pid container:my-dotnet-app \
  --mount=type=image,source=dhi.io/busybox,destination=/dbg,ro \
  dhi.io/dotnet:8 /dbg/bin/sh

Image variants

Docker Hardened Images come in different variants depending on their intended use.

Runtime variants are designed to run your application in production. These images are intended to be used either directly or as the FROM image in the final stage of a multi-stage build. These images typically:

• Run as the nonroot user
• Do not include a shell or a package manager
• Contain only the minimal set of libraries needed to run the app

Build-time variants (SDK variants) typically include -sdk in the variant name and are intended for use in the first stage of a multi-stage Dockerfile. These images typically:

• Run as the root user
• Include a shell and package manager
• Are used to build or compile applications
• Include the complete .NET SDK toolchain

Available variants include:

• 8: .NET 8 runtime only
• 8-sdk: .NET 8 SDK for development and building
• 9: .NET 9 runtime only
• 9-sdk: .NET 9 SDK for development and building

For ASP.NET Core application, use the separate DHI ASP.NET Core repository:

• dhi.io/aspnetcore:8: ASP.NET Core 8 runtime
• dhi.io/aspnetcore:9: ASP.NET Core 9 runtime

Migrate to a Docker Hardened Image

To migrate your application to a Docker Hardened Image, you must update your Dockerfile. At minimum, you must update the base image in your existing Dockerfile to a Docker Hardened Image. This and a few other common changes are listed in the following table of migration notes.

The following steps outline the general migration process.

1. Find hardened images for your app.

   A hardened image may have several variants. Inspect the image tags and find the image variant that meets your needs.

2. Update the base image in your Dockerfile.

   Update the base image in your application's Dockerfile to the hardened image you found in the previous step. For framework images, this is typically going to be an image tagged as -sdk because it has the tools needed to install packages and dependencies.

3. For multi-stage Dockerfiles, update the runtime image in your Dockerfile.

   To ensure that your final image is as minimal as possible, you should use a multi-stage build. All stages in your Dockerfile should use a hardened image.

   While intermediary stages will typically use images tagged as -sdk, your final runtime stage should use a runtime image variant.

4. Install additional packages.

   Docker Hardened Images contain minimal packages in order to reduce the potential attack surface. You may need to install additional packages in your Dockerfile. Inspect the image variants to identify which packages are already installed.

   Only images tagged as -sdk typically have package managers. You should use a multi-stage Dockerfile to install the packages. Install the packages in the build stage that uses a -sdk image. Then, if needed, copy any necessary artifacts to the runtime stage that uses a non-SDK image.

   For Alpine-based images, you can use apk to install packages. For Debian-based images, you can use apt-get to install packages.

Before and after migration

Before (using official images):

FROM mcr.microsoft.com/dotnet/sdk:8.0 AS build
WORKDIR /src
COPY . .
RUN dotnet publish -c Release -o /app

FROM mcr.microsoft.com/dotnet/aspnet:8.0
WORKDIR /app
COPY --from=build /app .
ENTRYPOINT ["dotnet", "MyApp.dll"]

After (using DHI):

FROM dhi.io/dotnet:8-sdk AS build
WORKDIR /src
COPY *.csproj .
RUN dotnet restore
COPY . .
RUN dotnet publish -c Release -o /app

FROM dhi.io/aspnetcore:8
WORKDIR /app
COPY --from=build /app .
EXPOSE 8080
ENTRYPOINT ["dotnet", "MyApp.dll"]

Troubleshooting migration

The following are common issues that you may encounter during migration.

General debugging

The hardened images intended for runtime don't contain a shell nor any tools for debugging. The recommended method for debugging applications built with Docker Hardened Images is to use Docker Debug⁠ to attach to these containers. Docker Debug provides a shell, common debugging tools, and lets you install other tools in an ephemeral, writable layer that only exists during the debugging session.

Permissions

By default image variants intended for runtime, run as the nonroot user. Ensure that necessary files and directories are accessible to the nonroot user. You may need to copy files to different directories or change permissions so your application running as the nonroot user can access them.

Privileged ports

Non-SDK hardened images run as a nonroot user by default. As a result, applications in these images can't bind to privileged ports (below 1024) when running in Kubernetes or in Docker Engine versions older than 20.10. To avoid issues, configure your application to listen on port 1025 or higher inside the container, even if you map it to a lower port on the host. For example, docker run -p 80:8080 my-image will work because the port inside the container is 8080, and docker run -p 80:81 my-image won't work because the port inside the container is 81.

No shell

By default, image variants intended for runtime don't contain a shell. Use -sdk images in build stages to run shell commands and then copy any necessary artifacts into the runtime stage. In addition, use Docker Debug to debug containers with no shell.

Entry point

Docker Hardened Images may have different entry points than images such as Docker Official Images. Use docker image inspect to inspect entry points for Docker Hardened Images and update your Dockerfile if necessary.

.NET-specific considerations

• Global tools: Global .NET tools should be installed during the build stage using the SDK image
• NuGet packages: All package restoration should happen in the SDK build stage
• Runtime dependencies: Ensure all required runtime libraries are present in the final runtime image
• Configurations: Use environment variables or configuration files that are accessible to the nonroot user
• File paths: When running applications with dotnet build, the DLL is located in bin/Debug/net8.0/ or bin/Release/net8.0/ subdirectories. When using dotnet publish, the DLL is placed in the specified output directory.
• Logging: Configure logging to write to stdout/stderr or to directories writable by the nonroot user
• Health checks: Implement health check endpoints for Kubernetes liveness and readiness probes