feat: add `docker_image` based hooks?

# 💡 Summary #

Addition of `docker_image` based hooks for formatting and validating `packer` configs.

## Motivation and context ##
The current hooks require an installation of `packer` to work. Those who utilize Docker/Podman can more easily setup and run the hooks without a system install.

This would be useful because it removes the need for a `packer` installation and adds more cross-platform support.

## Implementation notes ##

An example hook config might look something like this:

```yaml
- description: |
Runs `packer fmt` on appropriate files inside a Docker.
entry: ghcr.io/cisagov/pre-commit-packer:latest /opt/pre-commit-packer/hooks/packer_fmt.sh
files: \.pkr(?:vars)?\.hcl$
id: packer_fmt_docker
language: docker_image
name: Packer Format (Docker)
```
It would require a `Dockerfile`, building an image with GitHub Actions and adding it to GHCR or some other registry.

## Acceptance criteria ##

- [ ] Current `script` based hooks continue to work as expected
- [ ] New hooks work allow easier usage for Docker/Podman users