Scott Hanselman

Trying out new .NET Core Alpine Docker Images

November 22, '17 Comments [20] Posted in Docker | DotNetCore | Open Source
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Docker ContainersI blogged recently about optimizing .NET and ASP.NET Docker files sizes. .NET Core 2.0 has previously been built on a Debian image but today there is preview image with .NET Core 2.1 nightlies using Alpine. You can read about the announcement here about this new Alpine preview image. There's also a good rollup post on .NET and Docker.

They have added two new images:

  • 2.1-runtime-alpine
  • 2.1-runtime-deps-alpine

Alpine support is part of the .NET Core 2.1 release. .NET Core 2.1 images are currently provided at the microsoft/dotnet-nightly repo, including the new Alpine images. .NET Core 2.1 images will be promoted to the microsoft/dotnet repo when released in 2018.

NOTE: The -runtime-deps- image contains the dependancies needed for a .NET Core application, but NOT the .NET Core runtime itself. This is the image you'd use if your app was a self-contained application that included a copy of the .NET Core runtime. This is apps published with -r [runtimeid]. Most folks will use the -runtime- image that included the full .NET Core runtime. To be clear:

- The runtime image contains the .NET Core runtime and is intended to run Framework-Dependent Deployed applications - see sample

- The runtime-deps image contains just the native dependencies needed by .NET Core and is intended to run Self-Contained Deployed applications - see sample

It's best with .NET Core to use multi-stage build files, so you have one container that builds your app and one that contains the results of that build. That way you don't end up shipping an image with a bunch of SDKs and compilers you don't need.

NOTE: Read this to learn more about image versions in Dockerfiles so you can pick the right tag and digest for your needs. Ideally you'll pick a docker file that rolls forward to include the latest servicing patches.

Given this docker file, we build with the SDK image, then publish, and the result is about 219megs.

FROM microsoft/dotnet:2.0-sdk as builder  

RUN mkdir -p /root/src/app/dockertest
WORKDIR /root/src/app/dockertest

COPY dockertest.csproj .
RUN dotnet restore ./dockertest.csproj

COPY . .
RUN dotnet publish -c release -o published

FROM microsoft/dotnet:2.0.0-runtime

WORKDIR /root/
COPY --from=builder /root/src/app/dockertest/published .
ENV ASPNETCORE_URLS=http://+:5000
EXPOSE 5000/tcp
CMD ["dotnet", "./dockertest.dll"]

Then I'll save this as Dockerfile.debian and build like this:

> docker build . -t shanselman/dockertestdeb:0.1 -f dockerfile.debian

With a standard ASP.NET app this image ends up being 219 megs.

Now I'll just change one line, and use the 2.1 alpine runtime

FROM microsoft/dotnet-nightly:2.1-runtime-alpine

And build like this:

> docker build . -t shanselman/dockertestalp:0.1 -f dockerfile.alpine

and compare the two:

> docker images | find /i "dockertest"
shanselman/dockertestalp 0.1 3f2595a6833d 16 minutes ago 82.8MB
shanselman/dockertestdeb 0.1 0d62455c4944 30 minutes ago 219MB

Nice. About 83 megs now rather than 219 megs for a Hello World web app. Now the idea of a microservice is more feasible!

Please do head over to the GitHub issue here https://github.com/dotnet/dotnet-docker-nightly/issues/500 and offer your thoughts and results as you test these Alpine images. Also, are you interested in a "-debian-slim?" It would be halfway to Alpine but not as heavy as just -debian.

Lots of great stuff happening around .NET and Docker. Be sure to also check out Jeff Fritz's post on creating a minimal ASP.NET Core Windows Container to see how you can squish .(full) Framework applications running on Windows containers as well. For example, the Windows Nano Server images are just 93 megs compressed.


Sponsor: Get the latest JetBrains Rider preview for .NET Core 2.0 support, Value Tracking and Call Tracking, MSTest runner, new code inspections and refactorings, and the Parallel Stacks view in debugger.

About Scott

Scott Hanselman is a former professor, former Chief Architect in finance, now speaker, consultant, father, diabetic, and Microsoft employee. He is a failed stand-up comic, a cornrower, and a book author.

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Disclaimer: The opinions expressed herein are my own personal opinions and do not represent my employer's view in any way.