Create Asp Net Core Docker App

In this post, we will look at building ASP.NET Core Docker App. We will first examine how to do this without Visual Studio and then with Visual Studio Tools for Docker.

Prerequisites

• Docker for Windows
• Visual Studio 2017 with .NET Core cross-platform development Workloads

Without Visual Studio 2017

ASP.NET Core app can be coded entirely without IDE. In this section, we will examine how to dockerize a ASP.NET Core app.

Sample ASP.NET Core App

Open a command prompt and run the following command to create and test a sample ASP.NET Core app.

REM create a folder named 'HelloWorld'
mkdir HelloWorld

REM navigate into the newly created folder
cd HelloWorld

REM create a new ASP.NET Core console app based on the name of the folder
dotnet new webapi

REM run the newly created ASP.NET Core console app
dotnet run

We will see that the application is now listening on http://localhost:5000. Open your favorite web browser and navigate to the following url http://localhost:5000/api/values. Since we are running a web api, you should see a ["value1","value2"].

Dockerize ASP.NET Core App

To dockerize the ASP.NET Core app, we will create a Dockerfile, with no extension, and save it in the HelloWorld directory with the following content. Dockerfile contains Docker build instructions that runs in sequence.

# FROM instruction, which must be first, initializes a new build stage and sets the Base Image for the remaining instructions.
FROM microsoft/aspnetcore-build:2.0 AS build-env

# WORKDIR sets the working directory of any remaining RUN, CMD, ENTRYPOINT, COPY and ADD instruction.
WORKDIR /app

# COPY copies the csproj to the container.
COPY *.csproj ./

# RUN executes commands in a new layer on top of the current image and commit the results. In this case, we get the needed dependencies of the project.
RUN dotnet restore

# COPY copies the rest of the files into our container into new layers.
COPY . ./

# RUN executes the command to publish the app into the out directory.
RUN dotnet publish -c Release -o out

# We are using Docker multi-stage build feature, which is why we have another FROM
# FROM instruction for a runtime image
FROM microsoft/aspnetcore:2.0

# WORKDIR sets the working directory of any remaining RUN, CMD, ENTRYPOINT, COPY and ADD instruction.
WORKDIR /app

# COPY copies from the build-env image the content of the /app/out folder to the current image
COPY --from=build-env /app/out .

# ENTRYPOINT instruction allows the container to run as an executable.
ENTRYPOINT ["dotnet", "HelloWorld.dll"]

Build and Run the Docker App

With the dockerfile ready, we can execute the following commands to build and run the container.

docker build -t aspnetapp .
docker run -it --rm -p 5000:80 aspnetapp

Port 5000 is mapped to port 80 so you will get the same output if you access the url http://localhost:5000/api/values.

Using Visual Studio 2017

Visual Studio 2017 provides support for Docker via Visual Studio Tools for Docker.

Create Sample .NET Core App

In Visual Studio 2017, create a ASP.NET Core app. Run the app and you should see an output if you access the url http://localhost:5000/api/values.

Add Docker Support

Adding docker support in Visual Studio is extremely easily with Visual Studio Tools for Docker.

• Right click the project, select Add and Docker Support
• On the Docker Support Options prompt, select the proper target OS whether you are running Windows or Linux containers.

This would add a solution folder called docker-compose and several docker related files.

Build and Run the Docker App

With the docker-compose as StartUp project, simply run the application and you should see an output if you access the url http://localhost:5000/api/values. The port may be different, be sure to check the properties in the project.

Possible Errors

Did you mean to run dotnet SDK commands?

If you are getting the following message

Did you mean to run dotnet SDK commands? Please install dotnet SDK from:
http://go.microsoft.com/fwlink/?LinkID=798306&clcid=0x409

You need to double check that the .dll targeted by the run by ENTRYPOINT is right. Check the spelling, casing, and the path.