docker: include git commit hash in swarm version

[skylenet]

We need the .git directory during builds so that we can detect the git commit hash that is important to have.

Previously:

$ swarm version
Swarm                                                                        
Version: 0.4.2-unstable                                                               
Go Version: go1.12.5                                                                          
OS: linux   

Now:

$ swarm version
Swarm                                                                                                                            
Version: 0.4.2-unstable                                                                                                          
Git Commit: 8b37def9613e6021a865af0cc4ef42c20a66a1fb                                                                             
Go Version: go1.12.5                                                                                                             
OS: linux