Move Terraform docker image to the repo

[phillebaba]

The Docker image used to apply the Terraform modules lives in another repository. https://github.com/XenitAB/github-actions

Issues occur when we update dependency versions like tfsec and tflint here but not in the other image.

We should consider moving the image to this repository instead.

[bittrance]

The terraform support in github-actions has a wider audience than terraform-modules. It is used by customers in repositories that are not sync:ed from terraform-modules. I agree we need to do something, but moving it here will solve the problem only for XKS users.