Build a 0.x series image that always is on the latest patch release

It feels like the cadence of [Terraform releases](https://github.com/hashicorp/terraform/releases) is increasing. We haven't adopted 0.13 and the first 0.14 beta [was released](https://github.com/hashicorp/terraform/releases/tag/v0.14.0-beta1).

We usually add an image here for the latest Terraform version when we're starting a project. This leads to some weird stuff like gaps of 10 releases between images we maintain.

I don't see any reason why we wouldn't want all our projects to be on the latest patch release. I think we should publish a `quay.io/azavea/terraform:0.xx` image tag that always resolves to the latest patch release for a Terraform series. This would help us keep up, at least within a specific series.

Start by reviewing the Terraform releases linked in the issue and inspecting how this repository currently builds and publishes versioned images. Determine how a 0.xx tag could track the latest patch release in that series, then verify that the published tag resolves correctly as new patch releases appear.

Written by the indexing model from the issue text.