Add .NET Core SDK to pulumi/pulumi container

[chrsmith]

This PR updates the pulumi/pulumi docker image to include the .NET Core SDK. This will fix pulumi/actions#19 and allow you to run .NET-based Pulumi programs using GitHub Actions.

The PR also adds some Go-based integration tests to sanity check deploying Pulumi programs using the container. However, they are disabled by default since they require a PULUMI_ACCESS_TOKEN and I'm not sure if we have any way to avoid it getting disclosed into CI/CD logs.

Do we have a solution for this? As it would be nice to run these tests during scripts/build-and-publish-docker (after the containers have been built locally, but before they are pushed to DockerHub).

[justinvp]

Using the recently released 3.1 makes sense to me as that's the LTS version. However, we should probably move to using it everywhere else first, right? https://github.com/pulumi/scripts/blob/ae6f56d5f37fb5ac0bc232d746a768b137f21943/ci/install-common-toolchain.sh#L87-L99

[chrsmith]

I don't have any objection, but I'd love for @pgavlin to weigh in. Since one is the "runtime dependency for Pulumi apps users are running" and the other is a "developer-machine dependency" we are installing when building Pulumi locally.

Having the versions be different is probably asking for trouble. But it might also lead to unintended consequences. But upgrading the .NET SDK we use to the LTS version seems like a general thing we should be doing. Right?

[chrsmith]

I'll drop this down to 3.0 to match everything else for the time being, thanks Justin.

[pgavlin]

I'm not sure if we have any way to avoid it getting disclosed into CI/CD logs.

We already rely on Travis to blind secrets. I don't think we're adding risk by passing the token into the container.