Visual Studio Tools for Docker with ASP.NET Core
Visual Studio 2017 supports building, debugging, and running ASP.NET Core apps targeting either .NET Framework or .NET Core. Both Windows and Linux containers are supported.
Installation and setup
For Docker installation, review the information at Docker for Windows: What to know before you install and install Docker For Windows.
Shared Drives in Docker for Windows must be configured to support volume mapping and debugging. Right-click the System Tray's Docker icon, select Settings..., and select Shared Drives. Select the drive where Docker stores files. Select Apply.
Visual Studio 2017 versions 15.6 and later prompt when Shared Drives aren't configured.
Add Docker support to an app
The ASP.NET Core project's target framework determines the supported container types. Projects targeting .NET Core support both Linux and Windows containers. Projects targeting .NET Framework only support Windows containers.
When adding Docker support to a project, choose either a Windows or a Linux container. The Docker host must be running the same container type. To change the container type in the running Docker instance, right-click the System Tray's Docker icon and choose Switch to Windows containers... or Switch to Linux containers....
When creating a new app with the ASP.NET Core Web Application project templates, select the Enable Docker Support checkbox:
If the target framework is .NET Core, the OS drop-down allows for the selection of a container type.
The Visual Studio Tools for Docker don't support adding Docker to an existing ASP.NET Core project targeting .NET Framework. For ASP.NET Core projects targeting .NET Core, there are two options for adding Docker support via the tooling. Open the project in Visual Studio, and choose one of the following options:
- Select Docker Support from the Project menu.
- Right-click the project in Solution Explorer and select Add > Docker Support.
Docker assets overview
The Visual Studio Tools for Docker add a docker-compose project to the solution, containing the following:
- .dockerignore: Contains a list of file and directory patterns to exclude when generating a build context.
- docker-compose.yml: The base Docker Compose file used to define the collection of images to be built and run with
docker-compose run, respectively.
- docker-compose.override.yml: An optional file, read by Docker Compose, containing configuration overrides for services. Visual Studio executes
docker-compose -f "docker-compose.yml" -f "docker-compose.override.yml"to merge these files.
A Dockerfile, the recipe for creating a final Docker image, is added to the project root. Refer to Dockerfile reference for an understanding of the commands within it. This particular Dockerfile uses a multi-stage build containing four distinct, named build stages:
FROM microsoft/aspnetcore:2.0-nanoserver-1709 AS base WORKDIR /app EXPOSE 80 FROM microsoft/aspnetcore-build:2.0-nanoserver-1709 AS build WORKDIR /src COPY *.sln ./ COPY HelloDockerTools/HelloDockerTools.csproj HelloDockerTools/ RUN dotnet restore COPY . . WORKDIR /src/HelloDockerTools RUN dotnet build -c Release -o /app FROM build AS publish RUN dotnet publish -c Release -o /app FROM base AS final WORKDIR /app COPY --from=publish /app . ENTRYPOINT ["dotnet", "HelloDockerTools.dll"]
The Dockerfile is based on the microsoft/aspnetcore image. This base image includes the ASP.NET Core NuGet packages, which have been pre-jitted to improve startup performance.
The docker-compose.yml file contains the name of the image that's created when the project runs:
version: '3' services: hellodockertools: image: hellodockertools build: context: . dockerfile: HelloDockerTools\Dockerfile
In the preceding example,
image: hellodockertools generates the image
hellodockertools:dev when the app runs in Debug mode. The
hellodockertools:latest image is generated when the app runs in Release mode.
Prefix the image name with the Docker Hub username (for example,
dockerhubusername/hellodockertools) if the image will be pushed to the registry. Alternatively, change the image name to include the private registry URL (for example,
privateregistry.domain.com/hellodockertools) depending on the configuration.
Select Docker from the debug drop-down in the toolbar, and start debugging the app. The Docker view of the Output window shows the following actions taking place:
- The microsoft/aspnetcore runtime image is acquired (if not already in the cache).
- The microsoft/aspnetcore-build compile/publish image is acquired (if not already in the cache).
- The ASPNETCORE_ENVIRONMENT environment variable is set to
Developmentwithin the container.
- Port 80 is exposed and mapped to a dynamically-assigned port for localhost. The port is determined by the Docker host and can be queried with the
- The app is copied to the container.
- The default browser is launched with the debugger attached to the container using the dynamically-assigned port.
The resulting Docker image is the dev image of the app with the microsoft/aspnetcore images as the base image. Run the
docker images command in the Package Manager Console (PMC) window. The images on the machine are displayed:
REPOSITORY TAG IMAGE ID CREATED SIZE hellodockertools latest f8f9d6c923e2 About an hour ago 391MB hellodockertools dev 85c5ffee5258 About an hour ago 389MB microsoft/aspnetcore-build 2.0-nanoserver-1709 d7cce94e3eb0 15 hours ago 1.86GB microsoft/aspnetcore 2.0-nanoserver-1709 8872347d7e5d 40 hours ago 389MB
The dev image lacks the app contents, as Debug configurations use volume mounting to provide the iterative experience. To push an image, use the Release configuration.
docker ps command in PMC. Notice the app is running using the container:
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES baf9a678c88d hellodockertools:dev "C:\\remote_debugge..." 21 seconds ago Up 19 seconds 0.0.0.0:37630->80/tcp dockercompose4642749010770307127_hellodockertools_1
Edit and continue
Changes to static files and Razor views are automatically updated without the need for a compilation step. Make the change, save, and refresh the browser to view the update.
Modifications to code files requires compiling and a restart of Kestrel within the container. After making the change, use CTRL + F5 to perform the process and start the app within the container. The Docker container isn't rebuilt or stopped. Run the
docker ps command in PMC. Notice the original container is still running as of 10 minutes ago:
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES baf9a678c88d hellodockertools:dev "C:\\remote_debugge..." 10 minutes ago Up 10 minutes 0.0.0.0:37630->80/tcp dockercompose4642749010770307127_hellodockertools_1
Publish Docker images
Once the develop and debug cycle of the app is completed, the Visual Studio Tools for Docker assist in creating the production image of the app. Change the configuration drop-down to Release and build the app. The tooling produces the image with the latest tag, which can be pushed to the private registry or Docker Hub.
docker images command in PMC to see the list of images:
REPOSITORY TAG IMAGE ID CREATED SIZE hellodockertools latest 4cb1fca533f0 19 seconds ago 391MB hellodockertools dev 85c5ffee5258 About an hour ago 389MB microsoft/aspnetcore-build 2.0-nanoserver-1709 d7cce94e3eb0 16 hours ago 1.86GB microsoft/aspnetcore 2.0-nanoserver-1709 8872347d7e5d 40 hours ago 389MB
docker images command returns intermediary images with repository names and tags identified as <none> (not listed above). These unnamed images are produced by the multi-stage build Dockerfile. They improve the efficiency of building the final image—only the necessary layers are rebuilt when changes occur. When the intermediary images are no longer needed, delete them using the docker rmi command.
There may be an expectation for the production or release image to be smaller in size by comparison to the dev image. Because of the volume mapping, the debugger and app were running from the local machine and not within the container. The latest image has packaged the necessary app code to run the app on a host machine. Therefore, the delta is the size of the app code.