docker/arcyd/arcyd-dockerfile: add openssh-client
docker/arcyd/arcyd-dockerfile: add openssh-client

Git urls often come in the 'ssh' variety, so add support for this into
the image.

Handling of credentials is not addressed by this commit, although it was
tested by mapping a pre-populated volume to /root/.ssh, so that the
supplied URLs would 'just work'.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/docker-testshell.sh
    .. start a new instance with SSH creds mapped to /root/.ssh ..
    .. verify that fetch and push to SSH remotes work as expected ..

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