x/build: add playground to build.golang.org

[ysmolski]

Currently running trybots for x/playground produces this:
https://storage.googleapis.com/go-build-log/2d26a65f/linux-amd64_e35969e6.log

Add Playground to build.golang.org.
Make builders be able to test Playground in the its docker container.

[andybons]

You should also add it to build.golang.org 😄

[ysmolski]

@andybons, is there any good doc describing how trybots work besides https://go.googlesource.com/build/ ?

[andybons]

Good question. @bradfitz ?

[bradfitz]

Nope, not besides reading the coordinator code. But this is not a good starter project.

[ysmolski]

Something is not clear. We have to test Playground via it's docker container. But where should we run that? I think it does not make much sense to run it on many platforms. Also I fail to find repos tests in similar fashion, but running not a go test ./... but rather docker container which will take more than 10 minutes to build from scratch. I will dig further a bit later...

[bradfitz]

Yes, we have no way to do this right now. That's why it's not a good starter project. This will be a lot of work and require a number of changes across our build system.

[bradfitz]

Rough outline, though:

• make a new misc-playground builder (similar to misc-compile-*).
• make a new host type in x/build/dashboard/builders.go etc that's VM-based, not container-based. Probably can resurrect docker2boot, which we used to use before Kubernetes. (we don't want to use Kubernetes here because docker build on Kubernetes remains a mess: Figure out how to support docker build on Kubernetes kubernetes/kubernetes#1806 etc)
• make a "go test" friendly test that runs exec.Command("docker", "build") etc if os.Getenv("GO_BUILDER_NAME") == "misc-playground"
• add some special cases for which tests to run in dashboard/builders.go and cmd/coordinator, like some of the others clarifying which repos run on which builders.

[dlorenc]

We just open sourced a tool today to make docker build on kubernetes less of a mess: github.com/GoogleCloudPlatform/kaniko