Use docker hub

[soerenmartius]

This is a conceptual PR with that contains ongoing work and isn't finished or ready for review yet. Just asking for early feedback.

I moved the Dockerfile to a new build-tools repository. Also, I've set up the repository in docker hub and enabled auto-build also. The Makefile will now pull the image from hub.docker.com instead of building it locally.

To-Do:

• Move the generic Makefile targets to the build-tools repository and load them in each repositories Makefile individually.
• Think about security. ( unprivileged user in Dockerfile, Mount Volume as Read-Only )
• We should inject the plan file from the docker-run-terraform-plan command to the docker-run-terraform-apply command. At the moment, docker-run-terraform-apply creates it's own state file.
• If we load the generic Makefile targets from the build-tools repository, how can we prevent MITM attacks?

[soerenmartius]

ok, that works as expected already. The next step will be to move the Makefile to the build-tools repository and to include in the modules repository with curl.

Just try to run make or make help inside the directory.

The biggest question remains though: Can we get this to work on a windows powershell 👎

[soerenmartius]

rdy for final review @mariux