build: switch to scratch image

[aschaber1]

Description of your changes

Switched the base image from alpine to scratch reducing the image size and potential issues with dependency maintenance.

$ podman image ls terraform-docs
localhost/terraform-docs  scratch     c091ac42b71f  5 minutes ago   20.2 MB
localhost/terraform-docs  master      126a7fcbb8a6  10 minutes ago  25.7 MB

I have:

• Read and followed terraform-docs' contribution process.
• All tests pass when I run make test.

How has this code been tested

I used Podman to run container locally: podman run --volume $PWD:/terraform-docs terraform-docs:latest markdown /terraform-docs and it worked seamlessly.