ARM64 Support

[odidev]

Hi Team,
I am trying to use “gcs-resource” image over ARM64 but it seems it is not available.

I have updated below files to build and release an arm64 image:

In Makefile, I have used buildx to build and push the image for both platforms and also added an install tag to install buildx binary and created a new builder and switched to it.
In travis.yml added “dist:focal” and updated 'go' version to “1.12.6”.

Commit: odidev@72f4538

After the above changes the image was successfully uploaded in Dockerhub .

Dockerhub: https://hub.docker.com/repository/docker/odidev/gcs-resource

It will be helpful if the arm64 image is released. If interested, I will raise a PR with the above changes.