Unable to push nuGet packages to GitLab with dotnet CLI due to Unauthorized error

GitLab now supports nuget public and private feed repository. I've got a public project (e.g: https://gitlab.com/sunnyatticsoftware/sasw-test-support) I create an access token for my user with api and write_repository (e.g: AAABBBCCCDDD)

I create a group variable in my CI/CD: SASW_API_ACCESS_TOKEN: AAABBBCCCDDD. All normal.

Then I create the multi stage CI/CD script to build, pack and publish. When attempting to publish the nuGet package with the following: dotnet nuget push **/*.nupkg --source https://gitlab.com/api/v4/projects/17141695/packages/nuget/index.json --api-key AAABBBCCCDDD --skip-duplicate

I get the error:

info : Pushing Sasw.TestSupport.2.0.2.nupkg to 'https://gitlab.com/api/v4/projects/17141695/packages/nuget'...
info :   PUT https://gitlab.com/api/v4/projects/17141695/packages/nuget/
info :   Unauthorized https://gitlab.com/api/v4/projects/17141695/packages/nuget/ 397ms
error: Response status code does not indicate success: 401 (Unauthorized).
ERROR: Job failed: exit code 1

The documentation doesn't mention anything special, but I notice that when using the (legacy?) nuget CLI it passes a username. Dotnet CLI, however, doesn't support username, just API KEY.

Any idea why this is not working?

This is my CI/CD script:

variables:
  GITLAB_RUNNER_DOTNET_CORE: mcr.microsoft.com/dotnet/core/sdk:3.1
  NUGET_REPOSITORY: https://gitlab.com/api/v4/projects/${CI_PROJECT_ID}/packages/nuget/index.json
  NUGET_API_KEY: $SASW_API_ACCESS_TOKEN
  NUGET_FOLDER_NAME: nupkgs
  NUGET_VERSION_SUFFIX: $SASW_PRERELEASE_SUFFIX

stages:
  - build
  - pack
  - release
  
#Docker image
image: $GITLAB_RUNNER_DOTNET_CORE

#Jobs
ci:
  stage: build
  script:
    - dotnet restore --no-cache --force
    - dotnet build --configuration Release --no-restore
    #- dotnet vstest test/*UnitTests/bin/Release/**/*UnitTests.dll
    #- dotnet vstest test/*IntegrationTests/bin/Release/**/*IntegrationTests.dll
    
pack-prerelease:
  stage: pack
  script:
    - dotnet pack *.sln --configuration Release --output $NUGET_FOLDER_NAME --version-suffix $NUGET_VERSION_SUFFIX --include-symbols -p:SymbolPackageFormat=snupkg
  artifacts:
    paths:
    - $NUGET_FOLDER_NAME
    expire_in: 1 week
  except:
    - master

pack-release:
  stage: pack
  script:
    - dotnet pack *.sln --configuration Release --output $NUGET_FOLDER_NAME
  artifacts:
    paths:
    - $NUGET_FOLDER_NAME
    expire_in: 1 week
  only:
    - master

publish-nuget:
  stage: release
  script:
    - dotnet nuget push **/*.nupkg --source $NUGET_REPOSITORY --api-key $NUGET_API_KEY --skip-duplicate

PS: The project is public, so in case it's needed have a look at: https://gitlab.com/sunnyatticsoftware/sasw-test-support/-/jobs/451080235

UPDATE 1: Further verbosity from my local linux console

$ dotnet nuget -v Debug push **/*.nupkg --source https://gitlab.com/api/v4/projects/17141695/packages/nuget/index.json --api-key cBwt5_hidden_ --skip-duplicate
trace: NuGet Command Line Version: 5.4.0.2
info : Pushing Sasw.TestSupport.2.0.2.nupkg to 'https://gitlab.com/api/v4/projects/17141695/packages/nuget'...
info :   PUT https://gitlab.com/api/v4/projects/17141695/packages/nuget/
info :   Unauthorized https://gitlab.com/api/v4/projects/17141695/packages/nuget/ 1159ms
error: Response status code does not indicate success: 401 (Unauthorized).
trace: System.AggregateException: One or more errors occurred. (Response status code does not indicate success: 401 (Unauthorized).)
trace:  ---> System.Net.Http.HttpRequestException: Response status code does not indicate success: 401 (Unauthorized).
trace:    at System.Net.Http.HttpResponseMessage.EnsureSuccessStatusCode()
trace:    at NuGet.Protocol.Core.Types.PackageUpdateResource.EnsureSuccessStatusCode(HttpResponseMessage response, Nullable`1 codeNotToThrow, ILogger logger)
trace:    at NuGet.Protocol.Core.Types.PackageUpdateResource.<>c__DisplayClass23_0.<PushPackageToServer>b__0(HttpResponseMessage response)
trace:    at NuGet.Protocol.HttpSource.ProcessResponseAsync[T](HttpSourceRequest request, Func`2 processAsync, SourceCacheContext cacheContext, ILogger log, CancellationToken token)
trace:    at NuGet.Protocol.Core.Types.PackageUpdateResource.PushPackageToServer(String source, String apiKey, String pathToPackage, Int64 packageSize, Boolean noServiceEndpoint, Boolean skipDuplicate, TimeSpan requestTimeout, ILogger logger, CancellationToken token)
trace:    at NuGet.Protocol.Core.Types.PackageUpdateResource.PushPackageCore(String source, String apiKey, String packageToPush, Boolean noServiceEndpoint, Boolean skipDuplicate, TimeSpan requestTimeout, ILogger log, CancellationToken token)
trace:    at NuGet.Protocol.Core.Types.PackageUpdateResource.PushPackage(String packagePath, String source, String apiKey, Boolean noServiceEndpoint, Boolean skipDuplicate, TimeSpan requestTimeout, ILogger log, CancellationToken token, Boolean isSnupkgPush)
trace:    at NuGet.Protocol.Core.Types.PackageUpdateResource.Push(String packagePath, String symbolSource, Int32 timeoutInSecond, Boolean disableBuffering, Func`2 getApiKey, Func`2 getSymbolApiKey, Boolean noServiceEndpoint, Boolean skipDuplicate, SymbolPackageUpdateResourceV3 symbolPackageUpdateResource, ILogger log)
trace:    at NuGet.Commands.PushRunner.Run(ISettings settings, IPackageSourceProvider sourceProvider, String packagePath, String source, String apiKey, String symbolSource, String symbolApiKey, Int32 timeoutSeconds, Boolean disableBuffering, Boolean noSymbols, Boolean noServiceEndpoint, Boolean skipDuplicate, ILogger logger)
trace:    at NuGet.CommandLine.XPlat.PushCommand.<>c__DisplayClass0_1.<<Register>b__1>d.MoveNext()
trace:    --- End of inner exception stack trace ---
trace:    at System.Threading.Tasks.Task.ThrowIfExceptional(Boolean includeTaskCanceledExceptions)
trace:    at System.Threading.Tasks.Task`1.GetResultCore(Boolean waitCompletionNotification)
trace:    at System.Threading.Tasks.Task`1.get_Result()
trace:    at Microsoft.Extensions.CommandLineUtils.CommandLineApplication.<>c__DisplayClass56_0.<OnExecute>b__0()
trace:    at Microsoft.Extensions.CommandLineUtils.CommandLineApplication.Execute(String[] args)
trace:    at NuGet.CommandLine.XPlat.Program.MainInternal(String[] args, CommandOutputLogger log)

UPDATE 2: Alexey's answer is the proper one and up to date as per April 2021. GitLab has improved Nuget support in the last year and now it's possible to easily push packages to the repo package registry and have readacces on Nuget feed at project level or group level using deploy tokens.

Pushing NuGet packages built from the current repository is quite easy, following the documentation. You do not need a NuGet.config file to push packages, as the credentials can be specified for the dotnet push command. You also don't need to keep the credentials in the CI file, as the CI variables contain all the necessary temporary credentials to push packages to the project package registry.

This is a fragment from my working .gitlab-ci.yml file, which I literally copy-pasted from the docs. All the necessary information comes from CI variables, so this snippet is totally reusable.

nuget:
  stage: deploy
  image: mcr.microsoft.com/dotnet/sdk:5.0-buster-slim
  script:
    - dotnet pack -c Release -o $PWD/nuget
    - dotnet nuget add source "$CI_SERVER_URL/api/v4/projects/$CI_PROJECT_ID/packages/nuget/index.json" --name gitlab --username gitlab-ci-token --password $CI_JOB_TOKEN --store-password-in-clear-text
    - dotnet nuget push "$PWD/nuget/*.nupkg" --source gitlab
  only:
    - master
    - tags

Concerning the question about installing the package, you would, indeed, need a token. But it is not your personal access token. All you need really to let people install and restore from your feed is the repository deploy token. It doesn't have to be defined on the project level if you have multiple projects in a group, which contains several projects that have NuGet packages published. You can also create a deploy token for the whole group. The deploy token you create for this purpose only needs to have the read_package_registry permissions and it doesn't give any other rights to the users who get this token.

After creating a deploy token, the token name is used as the username, and the token itself is the password. You put both of those to the NuGet.config file, where you have your project or group feed listed.

For example:

<?xml version="1.0" encoding="utf-8"?>
<configuration>
    <packageSources>
        <clear />
        <add key="nuget.org" value="https://api.nuget.org/v3/index.json" protocolVersion="3" />
        <add key="myfeed" value="https://gitlab.mydomain.dev/api/v4/groups/19/-/packages/nuget/index.json" />
    </packageSources>
    <packageSourceCredentials>
        <myfeed>
            <add key="Username" value="gitlab+deploy-token-14" />
            <add key="ClearTextPassword" value="thetokenvalue" />
        </myfeed>
    </packageSourceCredentials>
</configuration>