Add Terraform 0.13 and 0.12

Based off zenika/terraform-azure-cli, build images using GitHub Actions and push to Docker Hub with a new tagging scheme. Currently, the latest tag is not being changed or updated.

1.0-alpha.4: Remove Terraform 0.14

There isn't a tagged version and only appears to be in the latest tag.
That's fine for now since we need to upgrade to 0.13 first, anyway.

1.0-alpha.3

Fix tag name for zenika image

1.0 Alpha 1

Testing the build process.

1.0-alpha.2

Testing image creation and pushing to Docker Hub.