Add an sshd container

This is mainly for testing.

You can run with something like
```
docker run -v ~/.ssh/id_rsa.pub:/root/.ssh/authorized_keys -p 2222:22 -e TINI_SUBREAPER=1 --pid=host mobylinux/sshd:36c44542d8120e384c724a078e3e489f7a58382c
```

if you want to run in the host pid namespace. You must provide an
authorized key, you may provide a host key.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

base/sshd/Dockerfile

@@ -0,0 +1,14 @@
+FROM alpine:edge
+
+RUN \
+  apk update && apk upgrade && \
+  apk add --no-cache \
+  openssh-server \
+  tini \
+  && true
+
+COPY . .
+
+RUN mkdir -p /etc/ssh /root/.ssh && chmod 0700 /root/.ssh
+
+CMD ["/sbin/tini", "/usr/bin/ssh.sh"]

base/sshd/Makefile

@@ -0,0 +1,29 @@
+.PHONY: tag push
+
+BASE=alpine:edge
+IMAGE=sshd
+
+default: push
+
+hash: Dockerfile etc/ssh/sshd_config usr/bin/ssh.sh etc/motd
+	DOCKER_CONTENT_TRUST=1 docker pull $(BASE)
+	tar cf - $^ | docker build --no-cache -t $(IMAGE):build -
+	docker run --rm $(IMAGE):build sh -c "cat $^ /lib/apk/db/installed | sha1sum" | sed 's/ .*//' > $@
+
+push: hash
+	docker pull mobylinux/$(IMAGE):$(shell cat hash) || \
+		(docker tag $(IMAGE):build mobylinux/$(IMAGE):$(shell cat hash) && \
+		 docker push mobylinux/$(IMAGE):$(shell cat hash))
+	docker rmi $(IMAGE):build
+	rm -f hash
+
+tag: hash
+	docker pull mobylinux/$(IMAGE):$(shell cat hash) || \
+		docker tag $(IMAGE):build mobylinux/$(IMAGE):$(shell cat hash)
+	docker rmi $(IMAGE):build
+	rm -f hash
+
+clean:
+	rm -f hash
+
+.DELETE_ON_ERROR:

base/sshd/etc/motd

@@ -0,0 +1 @@
+Welcome to Moby 

base/sshd/etc/ssh/sshd_config

@@ -0,0 +1,144 @@
+#	$OpenBSD: sshd_config,v 1.98 2016/02/17 05:29:04 djm Exp $
+
+# This is the sshd server system-wide configuration file.  See
+# sshd_config(5) for more information.
+
+# This sshd was compiled with PATH=/bin:/usr/bin:/sbin:/usr/sbin
+
+# The strategy used for options in the default sshd_config shipped with
+# OpenSSH is to specify options with their default value where
+# possible, but leave them commented.  Uncommented options override the
+# default value.
+
+#Port 22
+#AddressFamily any
+#ListenAddress 0.0.0.0
+#ListenAddress ::
+
+# The default requires explicit activation of protocol 1
+#Protocol 2
+
+# HostKey for protocol version 1
+#HostKey /etc/ssh/ssh_host_key
+# HostKeys for protocol version 2
+#HostKey /etc/ssh/ssh_host_rsa_key
+#HostKey /etc/ssh/ssh_host_dsa_key
+#HostKey /etc/ssh/ssh_host_ecdsa_key
+#HostKey /etc/ssh/ssh_host_ed25519_key
+
+# Lifetime and size of ephemeral version 1 server key
+#KeyRegenerationInterval 1h
+#ServerKeyBits 1024
+
+# Ciphers and keying
+#RekeyLimit default none
+
+# Logging
+# obsoletes QuietMode and FascistLogging
+#SyslogFacility AUTH
+#LogLevel INFO
+
+# Authentication:
+
+#LoginGraceTime 2m
+#PermitRootLogin prohibit-password
+#StrictModes yes
+#MaxAuthTries 6
+#MaxSessions 10
+
+#RSAAuthentication yes
+#PubkeyAuthentication yes
+
+# The default is to check both .ssh/authorized_keys and .ssh/authorized_keys2
+# but this is overridden so installations will only check .ssh/authorized_keys
+AuthorizedKeysFile	.ssh/authorized_keys
+
+#AuthorizedPrincipalsFile none
+
+#AuthorizedKeysCommand none
+#AuthorizedKeysCommandUser nobody
+
+# For this to work you will also need host keys in /etc/ssh/ssh_known_hosts
+#RhostsRSAAuthentication no
+# similar for protocol version 2
+#HostbasedAuthentication no
+# Change to yes if you don't trust ~/.ssh/known_hosts for
+# RhostsRSAAuthentication and HostbasedAuthentication
+#IgnoreUserKnownHosts no
+# Don't read the user's ~/.rhosts and ~/.shosts files
+#IgnoreRhosts yes
+
+# To disable tunneled clear text passwords, change to no here!
+PasswordAuthentication no
+#PermitEmptyPasswords no
+
+# Change to no to disable s/key passwords
+ChallengeResponseAuthentication no
+
+# Kerberos options
+#KerberosAuthentication no
+#KerberosOrLocalPasswd yes
+#KerberosTicketCleanup yes
+#KerberosGetAFSToken no
+
+# GSSAPI options
+#GSSAPIAuthentication no
+#GSSAPICleanupCredentials yes
+
+# Set this to 'yes' to enable PAM authentication, account processing,
+# and session processing. If this is enabled, PAM authentication will
+# be allowed through the ChallengeResponseAuthentication and
+# PasswordAuthentication.  Depending on your PAM configuration,
+# PAM authentication via ChallengeResponseAuthentication may bypass
+# the setting of "PermitRootLogin without-password".
+# If you just want the PAM account and session checks to run without
+# PAM authentication, then enable this but set PasswordAuthentication
+# and ChallengeResponseAuthentication to 'no'.
+#UsePAM no
+
+#AllowAgentForwarding yes
+#AllowTcpForwarding yes
+#GatewayPorts no
+#X11Forwarding no
+#X11DisplayOffset 10
+#X11UseLocalhost yes
+#PermitTTY yes
+#PrintMotd yes
+#PrintLastLog yes
+#TCPKeepAlive yes
+#UseLogin no
+#UsePrivilegeSeparation sandbox
+#PermitUserEnvironment no
+#Compression delayed
+#ClientAliveInterval 0
+#ClientAliveCountMax 3
+#UseDNS no
+#PidFile /run/sshd.pid
+#MaxStartups 10:30:100
+#PermitTunnel no
+#ChrootDirectory none
+#VersionAddendum none
+
+# no default banner path
+#Banner none
+
+# Default of no subsystems
+#Subsystem	sftp	/usr/lib/ssh/sftp-server
+
+# the following are HPN related configuration options
+# tcp receive buffer polling. disable in non autotuning kernels
+#TcpRcvBufPoll yes
+ 
+# disable hpn performance boosts
+#HPNDisabled no
+
+# buffer size for hpn to non-hpn connections
+#HPNBufferSize 2048
+
+
+# Example of overriding settings on a per-user basis
+#Match User anoncvs
+#	X11Forwarding no
+#	AllowTcpForwarding no
+#	PermitTTY no
+#	ForceCommand cvs server

base/sshd/usr/bin/ssh.sh

@@ -0,0 +1,6 @@
+#!/bin/sh
+
+KEYS=$(find /etc/ssh -name 'ssh_host_*_key')
+[ -z "$KEYS" ] && ssh-keygen -A >/dev/null 2>/dev/null
+
+exec /usr/sbin/sshd -D