Dockerize an ASP.NET Core application

Estimated reading time: 4 minutes

Introduction

This example demonstrates how to dockerize an ASP.NET Core application.

Why build ASP.NET Core?

Open-source
Develop and run your ASP.NET Core apps cross-platform on Windows, MacOS, and Linux
Great for modern cloud-based apps, such as web apps, IoT apps, and mobile backends
ASP.NET Core apps can run on .NET Core or on the full .NET Framework
Designed to provide an optimized development framework for apps that are deployed to the cloud or run on-premises
Modular components with minimal overhead retain flexibility while constructing your solutions

Prerequisites

This example assumes you already have an ASP.NET Core app on your machine. If you are new to ASP.NET you can follow a simple tutorial to initialize a project or clone our ASP.NET Docker Sample.

Create a Dockerfile for an ASP.NET Core application

Method 1:

Create a Dockerfile in your project folder.
Add the text below to your Dockerfile for either Linux or Windows Containers. The tags below are multi-arch meaning they pull either Windows or Linux containers depending on what mode is set in Docker Desktop for Windows. Read more on switching containers.
The Dockerfile assumes that your application is called aspnetapp. Change the Dockerfile to use the DLL file of your project.

FROM mcr.microsoft.com/dotnet/core/sdk:3.1 AS build-env
WORKDIR /app

# Copy csproj and restore as distinct layers
COPY *.csproj ./
RUN dotnet restore

# Copy everything else and build
COPY . ./
RUN dotnet publish -c Release -o out

# Build runtime image
FROM mcr.microsoft.com/dotnet/core/aspnet:3.1
WORKDIR /app
COPY --from=build-env /app/out .
ENTRYPOINT ["dotnet", "aspnetapp.dll"]

To make your build context as small as possible add a .dockerignore file to your project folder and copy the following into it.

bin/
obj/

Method 2 (build app outside Docker container):

Create a Dockerfile in your project folder.
Add the text below to your Dockerfile for either Linux or Windows Containers. The tags below are multi-arch meaning they pull either Windows or Linux containers depending on what mode is set in Docker Desktop for Windows. Read more on switching containers.
The Dockerfile assumes that your application is called aspnetapp. Change the Dockerfile to use the DLL file of your project. This method assumes that your project is already built and it copies the build artifacts from the publish folder. Refer to the Microsoft documentation on Containerize a .Net Core app.

The docker build step here will be much faster than method 1, as all the artifacts are built outside of the docker build step and the size of the base image is much smaller compared to the build base image.

This method is preferred for CI tools like Jenkins, Azure DevOps, GitLab CI, etc. as you can use the same artifacts in multiple deployment models if Docker isn’t the only deployment model being used. Addittionally, you’ll be able to run unit tests and publish code coverage reports, or use custom plugins on the artifacts built by the CI.
```
  FROM mcr.microsoft.com/dotnet/core/aspnet:3.1
  COPY bin/Release/netcoreapp3.1/publish/ App/
  WORKDIR /App
  ENTRYPOINT ["dotnet", "aspnetapp.dll"]
```
To make your build context as small as possible add a .dockerignore file to your project folder.

Build and run the Docker image

Open a command prompt and navigate to your project folder.
Use the following commands to build and run your Docker image:

$ docker build -t aspnetapp .
$ docker run -d -p 8080:80 --name myapp aspnetapp

View the web page running from a container

Go to localhost:8080 to access your app in a web browser.
If you are using the Nano Windows Container and have not updated to the Windows Creator Update there is a bug affecting how Windows 10 talks to Containers via “NAT” (Network Address Translation). You must hit the IP of the container directly. You can get the IP address of your container with the following steps:
1. Run docker inspect -f "{{ .NetworkSettings.Networks.nat.IPAddress }}" myapp
2. Copy the container IP address and paste into your browser. (For example, 172.16.240.197)