Explicitly install openssh-client in ruby_docker image

Because we will be doing git clone git@dev.gitlab.org, we need SSH. Recent images doesn't seem to have that installed by default.

Slack discussion starting at https://gitlab.slack.com/archives/C0X6W37EE/p1492792762820552

Related to https://gitlab.com/gitlab-org/omnibus-gitlab/issues/2258

marked as a Work In Progress

assigned to @balasankarc

mentioned in issue omnibus-gitlab#2258 (closed)

mentioned in merge request omnibus-gitlab!1504 (merged)

unmarked as a Work In Progress

QA image build I tried using this base image worked - https://dev.gitlab.org/gitlab/omnibus-gitlab/builds/1034461

assigned to @twk3

assigned to @marin

mentioned in commit 192d3ee8

merged