Make variable to specify the Terraform Provider Artifact Repo & Document specifying other download locations

[luebken]

What problem are you facing?

The Terraform provider I've been working on pushes it's binaries to GitHub.

@turkenh was so kind to point me to the right Dockerfile line: https://github.com/luebken/provider-aiven/pull/1/files

How could Upbound help solve your problem?

Document ${TERRAFORM_PROVIDER_DOWNLOAD_NAME} and an how to specify other downloads locations.

[ulucinar]

We had also better revisit the hardcoded repo root here and expose it as a Make variable so that it will be easier for folks to utilize other download locations (possibly without a need to change the Dockerfile manually), and we can then document this. cc. @ytsarev regarding our discussion on slack and your observations.

[ulucinar]

Added the enhancement label to reflect the Dockerfile & Makefile changes.

[ulucinar]

There's even a Terraform registry API which we can utilize to determine the download URL, which will simplify this task. In fact, it can just be enough to specify the native provider name & version (like it's sufficient for the Terraform CLI in the Terraform required_providers configuration block, i.e. with the source and version arguments, where source is not a URL but a provider name like hashicorp/aws). This would remove the burden of finding out the actual download location (URL) as long as the provider is registered.