Multi-stage builds allows for a single Dockerfile to define multiple steps to build a desired Docker Image. This can include using different base images for the different stages of the execution.

Task

Start writing the Dockerfile by defining the first stage of the Docker Build. The aim of this stage is to build the required .NET Core assembly in the out directory. This has been taken from our scenario on building ASP.NET Core Docker Containers

# First Stage
FROM microsoft/dotnet:1.1.1-sdk

RUN mkdir /build
WORKDIR /build

COPY dotnetapp.csproj .
RUN dotnet restore

COPY . .
RUN dotnet publish -c Release -o out

Once the first stage has completed, the Docker Build can use the artefacts as part of another build step. This second stage use a different base image, allowing for it to be optimized for production.

By changing the base image, it's possible to remove the SDK tooling, making it smaller and also lowering the attack surface.

Task

Extend the Dockerfile with the second stage to produce the desired production image.

# Second Stage
FROM microsoft/dotnet:1.1.1-runtime
WORKDIR /app
CMD ["dotnet", "dotnetapp.dll"]

COPY --from=0 /build/out /app/

With the Dockerfile created, you can use the Docker CLI to build the image in the same way as a standard Docekr Build.

Task

Create a Docker Image by building and executing the Dockerfile using the Docker CLI.

When creating the image we also define a friendly name and tag. The name should refer to the application, in this case aspnet. The tag is a string and commonly used as a version number, in this case it's v0.1.

docker build -t aspnet-app:v0.1 .

You can view the Docker Images on your host using docker images | head -n2

Without Mutli-Stage builds and optimizing the image, the deployed size would have been 894.5MB. With the image is now only 266MB.

If you receive the error, COPY --from=0 /build/out /app/ Unknown flag: from, it means you're running an older version of Docker without the multi-stage support. Step 1 of this scenario upgrades the current Docker version.

Once the Docker Image has been built you can be launch it in the same way as other Docker Images.

Task

Start the newly build image and expose the port 5000 so the web application can be accessed.

docker run -d \ -t -p 5000:5000 \ --name app \ aspnet-app:v0.1

Once the container and process has started you can use curl to access the running application.

You can view the application logs using docker logs app

curl http://localhost:5000

You've now successfully built a .NET application as a Docker Image.