Airgap

[fr33ky]

Hello, Here come some modifications so we can run estafette within an airgapped environment. A modificaiton is also provided to fix #40 with some new tests. As I didn't know how you build your go binary, I prefered to create a new multistage Dockerfile (Dockerfile.localbuild) that go test and go build and package. Sure the default value for repository will have to be updated to point to your own docker hub once merged / build, I guess.

Best regards