How To Execute Multiple Commands Per Dockerfile

April 04, 2021

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I dabbled into .NET recently and after finishing the project I ended up with 2 microservices which were .NET projects under the same solution. It turns out that when dockerizing the microservices their build instructions are identical. The only difference is the command which needs to be invoked in order to start the microservice.

Below you can see a sample Dockerfile for a .NET application with multiple microservices:

FROM mcr.microsoft.com/dotnet/sdk:5.0 AS build

# install necessary packages
COPY *.sln .
COPY MicroserviceA/*.csproj ./MicroserviceA/
COPY MicroserviceB/*.csproj ./MicroserviceB/
COPY Common/*.csproj ./Common/
RUN dotnet restore ./MySolution.sln

# build executables of all projects/microservices
COPY . ./
RUN dotnet publish -c Release -o out

# Copy executables
FROM mcr.microsoft.com/dotnet/aspnet:5.0
COPY --from=build /app/out .
CMD ["dotnet", "MicroserviceA.dll"]

As you can see the CMD directive invokes MicroserviceA, but what about MicroserviceB? Because all previous stages are the same for MicroserviceB it’s redundant to have another Dockerfile dedicated just to MicroserviceB.

Luckily, the CMD specified in the Dockerfile is only a default. This means that it can be overridden. Same logic goes for ENTRYPOINT directive.

This means that executing docker run myimage on the image built by the Dockerfile above will start MicroserviceA by default. However, executing docker run myimage dotnet MicroserviceB.dll will override the default CMD and invoke MicroserviceB instead. As a result the Dockerfile can be re-used for any microservice project in the .NET app.

An even cleaner solution is to use docker-compose command option in order to specify each microservice:

version: "3.9"
    image: myimage/microservicea
    build: .
    command: ["dotnet", "MicroserviceA.dll"]
    image: myimage/microserviceb
    build: .
    command: ["dotnet", "MicroserviceB.dll"]

I hope the post shed some light on overriding Dockerfile default CMD and on organizing your .NET apps!