Add support for handlebars in build-contexts #1275

Description

As of now, you can use {{defaultContext}} in a context expression. However, the same capability does not exist for build-contexts as the HandleBars invocation is absent.

As a workaround, I am embedding the Git reference directly, from this code: https://github.com/docker/actions-toolkit/blob/0a8b89fa07de1dd2de1da2590b6ce3f0378cca9d/src/context.ts#L54-L56

In my case, I am referencing shared code within a repo that contains many separate compilation and deployment targets that are not built into a single container.

Thanks!