Add docker-machine binary to Dockerfile

It seems like the gitlab/gitlab-runner:latest is missing docker-machine.. So I can't use the autoscalling feature.

Apr 29 13:08:14 foo docker[26212]: WARNING: Failed to update executor docker+machine for 8fe7ad88 exec: "docker-machine": executable file not found in $PATH
Apr 29 13:08:17 foo docker[26212]: WARNING: Failed to update executor docker+machine for 8fe7ad88 exec: "docker-machine": executable file not found in $PATH
Apr 29 13:08:20 foo docker[26212]: WARNING: Failed to update executor docker+machine for 8fe7ad88 exec: "docker-machine": executable file not found in $PATH
Apr 29 13:08:23 foo docker[26212]: WARNING: Failed to update executor docker+machine for 8fe7ad88 exec: "docker-machine": executable file not found in $PATH
Apr 29 13:08:26 foo docker[26212]: WARNING: Failed to update executor docker+machine for 8fe7ad88 exec: "docker-machine": executable file not found in $PATH
Apr 29 13:08:30 foo docker[26212]: WARNING: Failed to update executor docker+machine for 8fe7ad88 exec: "docker-machine": executable file not found in $PATH
Apr 29 13:08:33 foo docker[26212]: WARNING: Failed to update executor docker+machine for 8fe7ad88 exec: "docker-machine": executable file not found in $PATH