Docker .NET: 'The command could not be loaded ... No .NET SDKs were found'

The command could not be loaded, possibly because: ... No .NET SDKs were found. in Docker means you ran an SDK command such as dotnet build, dotnet publish or dotnet ef in an image that holds only the .NET runtime. The aspnet and runtime images run apps; they cannot build them. Build in mcr.microsoft.com/dotnet/sdk and copy the output into the runtime image.

That is the multi-stage Dockerfile: a FROM ...sdk:10.0 AS build stage that publishes, and a final stage on aspnet:10.0 or runtime:10.0 that copies the publish folder and runs dotnet YourApp.dll.

The error

docker run --rm --name fix8-bld-rt1 mcr.microsoft.com/dotnet/aspnet:10.0 dotnet build
The command could not be loaded, possibly because:
  * You intended to execute a .NET application:
      The application 'build' does not exist or is not a managed .dll or .exe.
  * You intended to execute a .NET SDK command:
      No .NET SDKs were found.

Download a .NET SDK:
https://aka.ms/dotnet/download

Learn about SDK resolution:
https://aka.ms/dotnet/sdk-not-found

The exit code was 155. dotnet ef in runtime:10.0 printed the same text with 'ef' in place of 'build', and a Dockerfile whose only stage was FROM mcr.microsoft.com/dotnet/aspnet:10.0 with RUN dotnet publish -c Release -o /app failed the build with exit code: 155 after the same lines.

Why it happens

dotnet is a small host program. Given a word that is not a .dll, it looks for an SDK to hand the command to. dotnet --info in the aspnet:10.0 image lists two runtimes, Microsoft.AspNetCore.App 10.0.13 and Microsoft.NETCore.App 10.0.13, and under SDKs says No SDKs were found. With no SDK, the host cannot tell whether you meant an app named build or an SDK command, so it prints both possibilities.

This is not the "You must install or update .NET" message, which appears when an app needs a runtime that is missing. Here the runtime is fine; the build tools were never in the image. dotnet ef in runtime:10.0 failed with the same message; it needs the SDK and, on top of it, the dotnet-ef tool installed in that stage.

The fix

FROM mcr.microsoft.com/dotnet/sdk:10.0 AS build
WORKDIR /src
COPY Hello/ .
RUN dotnet publish -c Release -o /app

FROM mcr.microsoft.com/dotnet/runtime:10.0
WORKDIR /app
COPY --from=build /app .
ENTRYPOINT ["dotnet", "Hello.dll"]

Built and run, this image printed Hello, World! and exited with 0. For a web app, make the final stage aspnet:10.0. On this machine docker image ls showed the SDK image at 1.25GB against 340MB for aspnet:10.0 and 300MB for runtime:10.0, which is why it stays in the build stage and never ships.

How it was reproduced

docker run of dotnet build in aspnet:10.0 and of dotnet ef in runtime:10.0, then a one-stage Dockerfile for a dotnet new console -n Hello project (net10.0), built with docker build --progress=plain -t fix8-bld-p5 -f fix8-bld-p5/Dockerfile fix8-bld-ctx. Images carried .NET 10.0.13; Docker Desktop engine 29.9.2. Container, tag and folder names are this test's own.

Frequently asked

Does the dotnet aspnet Docker image include the SDK?

No. mcr.microsoft.com/dotnet/aspnet and dotnet/runtime contain only runtimes. dotnet build and dotnet publish need mcr.microsoft.com/dotnet/sdk.

Can I run dotnet ef in the dotnet runtime Docker image?

No. dotnet ef in mcr.microsoft.com/dotnet/runtime:10.0 failed with No .NET SDKs were found. It needs the SDK and also the dotnet-ef tool installed in that stage.

What does No .NET SDKs were found mean?

The dotnet host found runtimes but no SDK, so it cannot run SDK commands. Install an SDK, or in Docker build in the sdk image and run in the runtime image.

More decoded errors in the Fixes category. If the app starts but asks for Microsoft.AspNetCore.App, see the runtime image post.