Compile dotnet core .net 4.5

Hello everybody for my first question.
I have a personnal project hosted on gitlab. It’s a dotnet core class library( C#) targeting both netcoreapp1.0 and net45 framework.
for now my .gitlac-ci.yml is working good but I need to restrict the build to netcoreapp1.0 till net45 causes the following error :

/usr/share/dotnet/sdk/1.0.1/Microsoft.Common.CurrentVersion.targets(1111,5): error MSB3644: The reference assemblies for framework ".NETFramework,Version=v4.5" were not found. To resolve this, install the SDK or Targeting Pack for this framework version or retarget your application to a version of the framework for which you have the SDK or Targeting Pack installed. Note that assemblies will be resolved from the Global Assembly Cache (GAC) and will be used in place of reference assemblies. Therefore your assembly may not be correctly targeted for the framework you intend. [/builds/b3b00/sly/sly/sly.csproj]

my csproj is

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFrameworks>netcoreapp1.0;net45</TargetFrameworks>
    <Description>#LY is a parser generator halfway between parser combinators and parser generator like ANTLR</Description>
	<Authors>b3b00</Authors>
	<PackageProjectUrl>https://github.com/b3b00/sly</PackageProjectUrl>
	<RepositoryUrl>https://github.com/b3b00/sly</RepositoryUrl>
	<PackageLicenseUrl>https://github.com/b3b00/sly/blob/master/LICENSE</PackageLicenseUrl>    
  </PropertyGroup>
  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Release|x86'" />
  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|AnyCPU'">
    <DocumentationFile></DocumentationFile>
    <LangVersion>6</LangVersion>
  </PropertyGroup>
  <ItemGroup>
    <Folder Include="parser\parser\llparser" />
    <Folder Include="parser\syntax" />
  </ItemGroup>
</Project>

and my gitlab-ci is

image : microsoft/dotnet:latest
stages:
  - build
before_script:
  - 'dotnet restore'
build:
 stage: build
 script:
  - 'dotnet build -c Release -f netcoreapp1.0 '
  - 'dotnet pack -c Release sly/sly.csproj'
 artifacts:
  paths:
  - sly/bin/Release/netcoreapp1.0/
  - expressionParser/bin/Release/netcoreapp1.0/
  - jsonparser/bin/Release/netcoreapp1.0/
  - sly/bin/Release/*.nupkg

does anybody suceeded in compiling dotnet core app for net framework 45 (or even lower)

thanks

Olivier