Add support for multiple exporters (#235)

Buildkit 0.13 introduced support for [multiple
exporters](https://docs.docker.com/build/exporters/#multiple-exporters).
We currently return an error in these situations.

Instead, inspect the builder's version when loading the node and relax
this error if we see it's running at least 0.13.

Fixes https://github.com/pulumi/pulumi-docker-build/issues/21.

CHANGELOG.md

@@ -1,10 +1,16 @@
 ## Unreleased
 
+### Added
+
+- Multiple exports are now allowed if the build daemon is detected to have
+  version 0.13 of Buildkit or newer. (https://github.com/pulumi/pulumi-docker-build/issues/21)
+
 ### Changed
 
 - Upgraded buildx from 0.16.0 to 0.18.0.
 
 ### Fixed
+
 - Custom `# syntax=` directives no longer cause validation errors. (https://github.com/pulumi/pulumi-docker-build/issues/300)
 
 ## 0.0.7 (2024-10-16)