### Is there an existing issue for this?

- [X] I have searched the existing issues

### Feature description

The use case I have in mind is to run Node 16.x Lambdas - the default container registry is `lambci/lambda` which is effectively abandoned. [Not even Node 14.x has been published](https://github.com/lambci/docker-lambda/pull/330). However, switching the registry over to [`amazon/aws-lambda-nodejs`](https://hub.docker.com/r/amazon/aws-lambda-nodejs) still fails to pull the container, since localstack is looking for the tag `nodejs16.x`. I'd like to be able to specify that if the runtime is `nodejs16.x` it just looks for tag `16`.

### 🧑‍💻 Implementation

_No response_

### Anything else?

_No response_
class TestDotNetCoreRuntimes:
    @pytest.mark.skipif(
        not use_docker(), reason="Dotnet functions only supported with docker executor"
    )
    @pytest.mark.parametrize(
        "zip_file,handler,runtime,expected_lines",
        [
            (
                TEST_LAMBDA_DOTNETCORE31,
                "dotnetcore31::dotnetcore31.Function::FunctionHandler",
                LAMBDA_RUNTIME_DOTNETCORE31,
                ["Running .NET Core 3.1 Lambda"],
            ),
            (
                TEST_LAMBDA_DOTNET6,
                "dotnet6::dotnet6.Function::FunctionHandler",
                LAMBDA_RUNTIME_DOTNET6,
                ["Running .NET 6 Lambda"],
            ),
        ],
        ids=["dotnetcore3.1", "dotnet6"],
    )
    @pytest.mark.skip_snapshot_verify
    def test_dotnet_lambda(
        self,
        zip_file,
        handler,
        runtime,
        expected_lines,
        lambda_client,