If it's just a matter of updating https://github.com/pulumi/actions I can provide a PR

Yes, we should definitely update that repo. See https://github.com/pulumi/actions/pull/6 and https://github.com/pulumi/actions/issues/10 for some more context. We probably just want to have docker://pulumi/actions

[:latest]

always pull the latest version, and have

[:vx.y.z]

pull a specific version of Pulumi. Does that sound right?

Yeah that sounds like a great approach to me