First use of Docker with ASP.Net Core

Now I have Docker up and running I wanted to set up an ASP.Net Core application. As with Docker itself, getting a simple “Hello, World” application running in the corporate environment was not as straightforward as I could have hoped. Below is the summary of the steps I had to take which will hopefully save other people some considerable time! I have pushed the source code up to my git repository and have made each incremental change a distinct commit so it is easy to follow.

Hello, world

First I started off with a very basic ASP.Net Core app by creating a new project in Visual Studio and accepting all the defaults. I didn’t add Docker support since I want to incrementally add it rather than jumping straight in with Docker Compose etc. My initial commit shows the basic app which can be run and tested via standard dotnet:

dotnet build
dotnet run

Browse to http://localhost:53545/api/values

Dockerfile

In order to build a Docker image in a repeatable, automated way, we can use a Dockerfile. This is a simple text document which starts with a base image (like a basic Ubuntu install or Windows Nanoserver) followed by a list of command-line instructions that assemble your image. A simple Dockerfile would look something like this one, a mix of two dotnet sample Dockerfiles for basic dotnet and aspnet:

FROM microsoft/aspnetcore-build:2.0
WORKDIR /app

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

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

ENTRYPOINT ["dotnet", "out/FirstDockerApp.dll"]

The Dockerfile is pretty self-explanatory (although see this post for more on the dotnet base docker images). I haven’t yet worked out why the nuget restore and source code copy are separate steps, though.

Building my application from the Docker command line runs this Dockerfile and gives me this output:

$ docker build
"docker build" requires exactly 1 argument.
See 'docker build --help'.

723254a2c089: Pull complete
abe15a44e12f: Pull complete
409a28e3cc3d: Pull complete
503166935590: Pull complete
09d35e02522b: Pull complete
7b915999d3d4: Pull complete
73625913309a: Pull complete
b457441733d9: Pull complete
2c97b35a16e4: Pull complete
ad5633a9a2eb: Pull complete
Digest: sha256:f905f7873db5fa5b6d67b326f717616595cb675b9e38bd9d307fe22f3769fc16
Status: Downloaded newer image for microsoft/aspnetcore-build:2.0             ]  41.09MB/153.4MB
 ---> 6105426f13e9loading [==>                                                ]  12.96MB/298.4MB
Step 2/7 : WORKDIR /appcomplete
Removing intermediate container 9b24ca1d3909                                  ]  40.55MB/153.4MB
 ---> bf35441e2363loading [==>                                                ]  12.42MB/298.4MB
Step 3/7 : COPY *.csproj ./
 ---> 863158a144a7ing
Step 4/7 : RUN dotnet restore
 ---> Running in 018f7409d033
  Restoring packages for /app/FirstDockerApp.csproj...
/usr/share/dotnet/sdk/2.1.4/NuGet.targets(103,5): error : Unable to load the service index for source https://api.nuget.org/v3/index.json. [/app/FirstDockerApp.csproj]
/usr/share/dotnet/sdk/2.1.4/NuGet.targets(103,5): error :   An error occurred while sending the request. [/app/FirstDockerApp.csproj]
/usr/share/dotnet/sdk/2.1.4/NuGet.targets(103,5): error :   Couldn't resolve host name [/app/FirstDockerApp.csproj]
The command '/bin/sh -c dotnet restore' returned a non-zero code: 1

I don’t have access to nuget.org through the corporate firewall. Next I’ll look at how to resolve this problem, and get my first Docker app up and running.

Written on January 19, 2018

docker (9) netcoreapp (10)