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.

Prerequisites

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.

Shared Drives

Tip

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....

New app

When creating a new app with the ASP.NET Core Web Application project templates, select the Enable Docker Support checkbox:

Enable Docker Support checkbox

If the target framework is .NET Core, the OS drop-down allows for the selection of a container type.

Existing app

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 build and 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.

Debug

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 Development within 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 docker ps command.
  • 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

Note

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.

Run the 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.

Run the 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

Note

The 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.