Error: It was not possible to find any installed .NET Core SDKs

For me it happened when I had wrong ENTRYPOINT in my DOCKERFILE

# Build runtime image
FROM mcr.microsoft.com/dotnet/core/aspnet:3.1
WORKDIR /app
COPY --from=build-env /app/out .
ENTRYPOINT ["dotnet", "SampleAppForDocker.dll"]

Make sure that you run correct dll in your ENTRYPOINT. I had wrong name of dll file.

I had this issue, but my ENTRYPOINT was correct. The issue was with stale or a corrupt mcr.microsoft.com/dotnet/core/sdk:3.1 image. So I purged everything and rebuilt the docker image.

For those unaware, this is how to do that:

Remove all docker containers: docker rm -f $(docker ps -a -q)
Remove all docker images:     docker rmi -f $(docker images)

After that, it worked fine.

In case of linux, the case sensitivity is important. In my case the problem was with (btw. it was working as Windows container)

ENTRYPOINT ["dotnet", "backendapi.dll"]

as the library name was written with different case. It was fixed once the entrypoint got named correctly as

ENTRYPOINT ["dotnet", "BackendAPI.dll"]

I followed the .Net Core app tutorial given by Microsoft, and ran into the same issue. I had the docker file set up as:

FROM mcr.microsoft.com/dotnet/aspnet:3.1
COPY bin/release/netcoreapp3.1/publish/ App/
WORKDIR /App
ENTRYPOINT ["dotnet", "NetCore.Docker.dll"]

but it did not pull the mcr.microsoft.com/dotnet/aspnet image to base my build on (I'm assuming, because I'm not an expert on this matter yet). I did a pull request for the aspnet image:

docker pull mcr.microsoft.com/dotnet/aspnet:3.1

and it created this image. I then built my project's docker image, and created my container. My container and app functioned as intended after the abovementioned.

In my case it was the problem of working directory. In Dockerfile you should be in the same working directory where your execution dll is placed. As follows : Dockerfile

On my case it was a space after double quotes:

ENTRYPOINT ["dotnet", " MyAssembly.dll"]

After removing it it worked.

In my case it was not related to docker. The "arguments" attribute in web.config had an error, the name of dll was incorrect. HTH.

Ensure your dll is copied from the publishing location to /app. Maybe your publishing configuration overrides that.

As suggested by @KarlGreen, ENTRYPOINT ["ls", "."] shows you if the dll was actually copied to the /app folder. In my case it wasn't, ls only listed web.config and wwwroot. If I published the Debug configuration instead, then the dll would be copied so there's some difference in publishing configuration, although I'm not sure what exactly. The output option to the publish command was not honored, but overridden by something else unknown to me.

I worked around it by explicitly copying the release output with a copy command e.g.

COPY --from=build-env /app/bin/Release/netcoreapp3.1 .

For me is I forgot to copy published folder from the build step into the final step.

...
FROM build AS publish
RUN dotnet publish  --configuration Release --output /dist

FROM base AS final
COPY --from=publish /dist .
...

I had the same error and as it turned out, I had accidentally mounted the application path of the container to the host system:

# Dockerfile
# ...
WORKDIR /var/myapp
# ...

# docker-compose.yml
services:
  myapp:
    // ...
    volumes:
      - /c/docker/myapp:/var/myapp     // <--- Had to be removed
    ports:
      - "80:5000"

Changing the volumes mapping resolved the issue.