Dockerize dd2tf

[toozej]

Creates a Docker image for dd2tf so it can be built and ran on a host which doesn't have Go installed (but of course does have Docker installed). Also includes some small Bash wrappers for quickly creating and running the Docker image, and a minor change to main.go to place exported Terraform files into a subdirectory called "exports" which is mounted to the Docker container.

[amnk]

@toozej Thank you very much for PRs. I know I've been silent on this, but let's merge this if dd2tf is still useful for someone. Would you be able to restore namings in code and squash your changes?