Add support to build for multiple platforms

[kolaente]

One of the main advantages of using buildx is cross platform builds and multiarch images. It would be awesome if this drone plugin could support it, like these forks do:

https://github.com/thegeeklab/drone-docker-buildx
https://github.com/owncloud-ci/drone-docker-buildx

[cblakkan]

It seems like the support exists, the setting is just platform: not platforms: . However when I try using this via plugins/buildx-ecr it doesn't invoke the docker buildx create commands before running docker buildx build and I end up getting the following error:

ERROR: Multi-platform build is not supported for the docker driver.
Switch to a different driver, or turn on the containerd image store, and try again.
Learn more at https://docs.docker.com/go/build-multi-platform/
exit status 1

[cblakkan]

Just a followup I got this to work by tricking the plugin to create a new builder even though it appears as though it should be doing so by default.

Setting the builder_driver option to the non default one causes buildx create to be invoked before buildx build

platform: linux/amd64,linux/arm64
builder_driver: docker-container