Remove Dockerfile templating in main build

[JulianCBC]

Build args can be used to specify the version of containers in FROM lines, so skip a step in the build process by passing the Terraform version in as an argument instead of templating the Dockerfile.

I haven't done the same conversion to the EKS image as it doesn't build at present.

This conflicts with #27 so I'll sort that out once one of these is merged.