docker build error: The current .NET SDK does not support targeting .NET Core 2.1.

[bbgw]

h3. Background
As a know-nothing-about-C#, I was trying to play with PlatformTM, but the captured error was thrown.

h3. Reproduce Steps and Symptom

[mbp:~/ref/PlatformTM] (master)
$ docker-compose up -d
Building ptm-server
Step 1/21 : FROM microsoft/aspnetcore:2.0 AS base
 ---> db030c19e94b
Step 2/21 : WORKDIR /app
 ---> Using cache
 ---> 2643110f194d
Step 3/21 : EXPOSE 80
 ---> Using cache
 ---> 04a9a170a9a2
Step 4/21 : FROM microsoft/aspnetcore-build:2.0 AS build
 ---> 06a6525397c2
Step 5/21 : WORKDIR /src
 ---> Using cache
 ---> b40247d12083
Step 6/21 : COPY PlatformTM.sln ./
 ---> Using cache
 ---> e90360af7559
Step 7/21 : COPY PlatformTM.API/PlatformTM.API.csproj PlatformTM.API/
 ---> Using cache
 ---> 507b0cdc3556
Step 8/21 : COPY PlatformTM.Core/PlatformTM.Core.csproj PlatformTM.Core/
 ---> Using cache
 ---> 5da81c42cb9c
Step 9/21 : COPY PlatformTM.Services/PlatformTM.Services.csproj PlatformTM.Services/
 ---> Using cache
 ---> 4d183c32a8be
Step 10/21 : COPY PlatformTM.Data/PlatformTM.Data.csproj PlatformTM.Data/
 ---> Using cache
 ---> 03a612f046f3
Step 11/21 : RUN dotnet restore -nowarn:msb3202,nu1503
 ---> Running in 1cb946033495
/usr/share/dotnet/sdk/2.1.202/Sdks/Microsoft.NET.Sdk/build/Microsoft.NET.TargetFrameworkInference.targets(135,5): error : The current .NET SDK does not support targeting .NET Core 2.1.  Either target .NET Core 2.0 or lower, or use a version of the .NET SDK that supports .NET Core 2.1. [/src/PlatformTM.Core/PlatformTM.Core.csproj]
/usr/share/dotnet/sdk/2.1.202/Sdks/Microsoft.NET.Sdk/build/Microsoft.NET.TargetFrameworkInference.targets(135,5): error : The current .NET SDK does not support targeting .NET Core 2.1.  Either target .NET Core 2.0 or lower, or use a version of the .NET SDK that supports .NET Core 2.1. [/src/PlatformTM.Data/PlatformTM.Data.csproj]
/usr/share/dotnet/sdk/2.1.202/Sdks/Microsoft.NET.Sdk/build/Microsoft.NET.TargetFrameworkInference.targets(135,5): error : The current .NET SDK does not support targeting .NET Core 2.1.  Either target .NET Core 2.0 or lower, or use a version of the .NET SDK that supports .NET Core 2.1. [/src/PlatformTM.Services/PlatformTM.Services.csproj]
/usr/share/dotnet/sdk/2.1.202/Sdks/Microsoft.NET.Sdk/build/Microsoft.NET.TargetFrameworkInference.targets(135,5): error : The current .NET SDK does not support targeting .NET Core 2.1.  Either target .NET Core 2.0 or lower, or use a version of the .NET SDK that supports .NET Core 2.1. [/src/PlatformTM.API/PlatformTM.API.csproj]
ERROR: Service 'ptm-server' failed to build: The command '/bin/sh -c dotnet restore -nowarn:msb3202,nu1503' returned a non-zero code: 1