GitLab multi-arch support with docker image

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Multi-arch is now fully supported on DockerHub. GitLab-EE really should support architectures other than x86_64 using the >=v2.2 Docker image spec and Manifest lists.

https://blog.docker.com/2017/11/multi-arch-all-the-things/

E.g. 
$ docker manifest inspect alpine | grep architecture 
"architecture": "amd64", 
"architecture": "arm", 
"architecture": "arm64", 
"architecture": "386", 
"architecture": "ppc64le", 
"architecture": "s390x",

$ docker manifest inspect gitlab/gitlab-ee | grep architecture | wc -l 
0

From ZD: https://gitlab.zendesk.com/agent/tickets/101163 (internal only)

cc: @marin